Kid, one of twenty-four found in chest (3)
I believe the answer is:
rib
This clue is a double definition.
'kid' is the first definition.
(I've seen this before)
'one of twenty-four found in chest' is the second definition.
(humans typically have 24 ribs)
(Other definitions for rib that I've seen before include "Bone; supporting member" , "Kid" , "Part of vault" , "Hinged rod in umbrella" , "Tease (sl.)" .)