Eleven small fish (4)
I believe the answer is:
side
'eleven' is the definition.
(a football side)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'ide' (type of fish).
's'+'ide'='SIDE'
(Other definitions for side that I've seen before include "A quarter, or half" , "Neighbourhood" , "Dies (anag)" , "Team; facet" , "Dies for own team" .)