Feeble chap, somewhat yellow, I'm pusillanimous (4)
I believe the answer is:
wimp
'feeble chap' is the definition.
'somewhat yellow i'm pusillanimous' is the wordplay.
'somewhat' indicates a hidden word.
'WIMP' is hidden in the letters of 'yellow im pusillanimous'.
(Other definitions for wimp that I've seen before include "Feeble or ineffective person" , "Drip" , "Feeble or ineffectual person" , "An ineffectual male" , "Spineless man" .)