Bird with large chest (4)
I believe the answer is:
lark
'bird' is the definition.
(lark is a kind of bird)
'with large chest' is the wordplay.
'with' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'chest' becomes 'ark' (historical term for a chest).
'l'+'ark' is 'LARK'.
(Other definitions for lark that I've seen before include "Fool" , "Mischievous game" , "Foolish activity, songbird" , "carefree adventure" , "Amusing escapade; bird" .)