Hold tightly and firmly (6)
I believe the answer is:
clench
'hold tightly and firmly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clench that I've seen before include "Grip tightly" , "Squeeze together tightly" , "Close firmly together" , "Grip; grit (teeth)" , "Sharply contract (muscle)" .)