Thanks to Goliath for a brilliant challenge.
Dependably enjoyable and witty. 19 across is a genius clue.
There may be more theme-related solutions in the grid, but I’m afraid I’m not familiar with the poem. If you are, let us know in the comments.

FAN (cool) + CLUB (bat)
SWAN (water bird) and SEA (water)
Double definition
Lupin is both a type of flower, and a bean (from 4 down’s solution)
AIRS (refreshes) + TRIKE (vehicle)
OPENS (starts) + [Googl]E (at last) + SAME (identical)
U[n]S[t]E[a]D[y] (occasionally)
PARADE (march) in which I’S (one’s) + LOST (gone astray)
READY* (*ground) to welcome LETTER (landlord) + (invite]D (finally)
V.I.P.S< (high and mighty people, <rejected)
(IN ITS PLACE)* (*shaped)
[B]LIGHTY EAR[thenware] (from)
[f]ORTNI[ght]< (<back, in a)
RAP (kind of music) + IDLY (with nonchalance)
(A BOY TEN)* (*playing)
Double definition
2. Family member’s ‘bon voyage’ at last, with a sigh of relief (6)
[bo]N [voyag]E (at last) with PHEW (a sigh of relief)
LANDS (secures) + CAPES (mantles)
SCRUM (rugby formation) + (MR (mister); I.E. (that is) welcomed)
[Sc]OTLA[nd]< (‘s,<rising)
The apostrophe is important in this case as it denotes possession and acts as the indicator of a ‘hidden inside’ clue.
SLIP (error) + SHOD (with footwear)
(ONE ACTED)* (*out)
E.G. (for one) appearing in NATIVITY (this theatrical production)
ILL (not well) kept by ARTERY (vessel)
(SE[a] TR[a]WLER)* (dismissed As, *out of order)
Double definition
(OT (part of the Bible, Old Testament) + L (left))< (wrapped, <up) in MIN (30 seconds)
John Milton famously wrote ‘Paradise Lost’, an epic poem across 12 books concerning the biblical fall of man
R (Romeo, NATO alphabet) + TROPE< (literary cliche, <coming up)
BUSH* (*kind of) that’s about R (right) – &lit
STUD[ents] (50% of)
This puzzle set the needle soaring on my laugh-o-meter, starting with 1a/4, once I’d nixed the idea of ‘gust(o)’ somewhere in the answer. It was one of several clues earning double ticks today, along with SCRUMMIER, LANDSCAPES, OPEN SESAME and PLASTICINE (for evoking happy childhood memories).
I was, in fact, rather profligate with the swooshy ticks but then Goliath is always a reliable source of fun with tricksy wordplay (very sneaky ‘A’s in WRESTLER), fine surfaces (PARADISE) and good humour (the tastier rugby of 5).
Thanks to Goliath for a welcome ray of sunshine on a tropical, thundery day and to Oriel whose happy task it was to unpick the clues.
I couldn’t see anything else to add to the Milton clues but it has been many years since I read it.
Struggled for a while with 19d, but only because I’d misspelt PLASTICENE, without checking the acronym.
I learnt that a lupin is also a bean.
Nothing too taxing, and plenty of smiles.
I agree with Oriel and Diane@1.
My memory of the poem has faded too but I do remember the couplet from A Shropshire Lad praising beer:
Malt does more than Milton can
To justify God’s way to man
Thanks Goliath and Oriel