Small champion loves large swimming group (6)
I believe the answer is:
school
'swimming group' is the definition.
(group of fish)
'small champion loves large' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'champion' becomes 'ch' (I've seen this in other clues).
'loves' becomes 'oo' (love means 0 in tennis).
'large' becomes 'L' (eg in clothes sizes).
's'+'ch'+'oo'+'l'='SCHOOL'
(Other definitions for school that I've seen before include "Train - a group of fish" , "Train (of thought?)" , "these like-minded people could follow" , "some bass?" , "classes?" .)