Firm hold or grasp (4)
I believe the answer is:
grip
I believe this is a double definition.
'firm hold' is the first definition.
(I've seen this before)
'grasp' is the second definition.
(synonyms)
(Other definitions for grip that I've seen before include "Keep firm hold of" , "Control" , "Clench" , "Purchase" , "member of camera crew" .)