There's a catch in being flat-chested (5)
I believe the answer is:
latch
'there's a catch' is the definition.
(latch is a kind of catch)
'in being flat-chested' is the wordplay.
'in' indicates a hidden word.
'LATCH' is hidden in the letters of 'being flat chested'.
(Other definitions for latch that I've seen before include "Something to secure" , "Gate fastening" , "Device to hold door shut" , "Door fastener" , "closer" .)