Small part played by fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(I know that sprat is a type of forage fish)
'small part played by' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'played' is an anagram indicator (letters playing around).
'by' says to put letters next to each other.
'part' with letters rearranged gives 'prat'.
's'+'prat'='SPRAT'
(Other definitions for sprat that I've seen before include "Young herring" , "Parts of a very small fish" , "Type of herring - small person or thing" , "Small herring-type fish" , "'Set a . . . . . to catch a herring'" .)