Cossack holding bag (3)
I believe the answer is:
sac
'bag' is the definition.
(a small bag)
'cossack holding' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'SAC' is hidden within 'cossack'.
(Other definitions for sac that I've seen before include "Bodily cavity enclosed by a membrane" , "Bag-like part in animal or plant" , "pocket" , "Animal or vegetable pouch" , "Bodily pouch" .)