Wrong to swallow hard bone (4)
I believe the answer is:
shin
'bone' is the definition.
(shin is a kind of bone)
'wrong to swallow hard' is the wordplay.
'wrong' becomes 'sin' (I've seen this before).
'to swallow' means one lot of letters goes inside another.
'hard' becomes 'h'.
'sin' enclosing 'h' is 'SHIN'.
(Other definitions for shin that I've seen before include "Body part" , "Front part of the leg between knee and ankle" , "kick" , "Front end of the tibia" , "Part of the human leg" .)