Bag of grouse mostly (4)
I believe the answer is:
grip
'bag' is the definition.
(I know that bag can be written as grip)
'grouse mostly' is the wordplay.
'grouse' becomes 'gripe' (synonyms).
'mostly' means to remove the last letter.
'gripe' with its final letter removed is 'GRIP'.
'of' acts as a link.
(Other definitions for grip that I've seen before include "Clench" , "mastery of problem" , "Hold hard" , "Grab" , "Grasp, clutch" .)