Fish often changing in part (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
'often changing in part' is the wordplay.
'in part' indicates a hidden word.
'TENCH' can be found hidden inside 'often changing'.
(Other definitions for tench that I've seen before include "Swimmer" , "catch" , "Fish of the carp family" , "Coarse fish" , "something fishy" .)