Bag in the boot, we hear! (3)
I believe the answer is:
sac
'bag' is the definition.
(a small bag)
'boot we hear' is the wordplay.
'boot' becomes 'sack' (I've seen this before).
'we hear' shows a homophone (sound like).
'sack' sounds like 'SAC'.
'in the' is the link.
(Other definitions for sac that I've seen before include "Pouch in the body" , "Distended membrane; pouch-like cavity" , "Bag-like part in animal or plant" , "Bag, pouch; cyst" , "Bodily cavity enclosed by a membrane" .)