What comes before play in pool -- urge to swim (8)
I believe the answer is:
prologue
'what comes before play' is the definition.
(a play may have a prologue)
'pool urge to swim' is the wordplay.
'to swim' is an anagram indicator.
'pool'+'urge'='poolurge'
'poolurge' anagrammed gives 'PROLOGUE'.
'in' acts as a link.
(Other definitions for prologue that I've seen before include "Introductory scene" , "Introductory passage" , "Start to play" , "Dramatic preface" , "Introductory piece" .)