Fish’s small head (7)
I believe the answer is:
snapper
'fish's' is the definition.
(snapper is a kind of fish)
'small head' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'head' becomes 'napper' (informal term for someone's head).
's'+'napper'='SNAPPER'
(Other definitions for snapper that I've seen before include "Fish; photographer" , "(Often red) food fish" , "sharp-toothed fish" , "Person photographing" , "Fish; marine turtle" .)