Grip firmly (5)
'grip firmly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clamp that I've seen before include "that stops one from driving?" , "squeeze" , "Grip firmly" , "Fix firmly" , "device to hold" .)
(Other definitions for grasp that I've seen before include "Get hold of firmly" , "Catch on" , "Appreciation" , "Take firm hold" , "Firmly grip" .)
(Other definitions for clutch that I've seen before include "Hold the car mechanism tight" , "Hold tight to the eggs" , "Grip tightly for car engine part" , "Grasp firmly, like engaging car part" , "Grasp - eggs - car pedal" .)