Swimmer left in swimming pool couldn't know to begin with (7)
I believe the answer is:
pollock
'swimmer' is the definition.
(type of fish)
'left in swimming pool couldn't know to begin with' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'to begin with' indicates taking the first letters.
The first letters of 'couldnt know' is 'ck'.
'pool' is an anagram of 'polo'.
'l' put within 'polo' is 'pollo'.
'pollo'+'ck'='POLLOCK'
(Other definitions for pollock that I've seen before include "See 9" , "Fish" , "Artist" .)