Grip shellfish with power (5)
I believe the answer is:
clamp
'grip' is the definition.
(I've seen this in another clue)
'shellfish with power' is the wordplay.
'shellfish' becomes 'clam' (clam is a kind of shellfish).
'with' means one lot of letters go next to another.
'power' becomes 'p' (symbol in physics).
'clam'+'p'='CLAMP'
(Other definitions for clamp that I've seen before include "Secure - store for potatoes" , "Wheel immobiliser" , "Heap of potatoes" , "Strong fastener" , "that stops one from driving?" .)