Fire mostly in cavity (3)
I believe the answer is:
sac
'cavity' is the definition.
(sac is a kind of cavity)
'fire mostly' is the wordplay.
'fire' becomes 'sack' (dismiss someone from their job).
'mostly' means to remove the last letter (most but not all of the word).
'sack' with its final letter taken off is 'SAC'.
'in' is the link.
(Other definitions for sac that I've seen before include "Bodily pouch" , "Bag-like part of the body" , "Bodily cavity enclosed by a membrane" , "Pouchlike structure" , "Animal or vegetable pouch" .)