Fellow pupil’s swimming group taking winning position (10)
I believe the answer is:
schoolmate
'fellow pupil's' is the definition.
(schoolmates are pupils at school together)
'swimming group taking winning position' is the wordplay.
'swimming group' becomes 'school' (group of fish).
'taking' means one lot of letters go next to another.
'winning position' becomes 'mate' (as in checkmate).
'school'+'mate'='SCHOOLMATE'
(Other definitions for schoolmate that I've seen before include "Young friend" , "Fellow pupil" , "fishwife?" , "childhood chum" , "Friends Reunited contact" .)