Organ transplant's helping bone cavities (5)
I believe the answer is:
antra
'bone cavities' is the definition.
(I've seen this before)
'organ transplant's helping' is the wordplay.
'helping' says the answer is hidden in the clue (a helping or portion of the letters).
'ANTRA' is hidden in the letters of 'organ transplants'.
(Other definitions for antra that I've seen before include "Natural cavities in bone; ratan (anag.)" , "Hollow spaces" , "Cavities - a rant (anag)" , "Cave!" , "Chambers" .)