Cook, as clams (5)
I believe the answer is:
steam
'cook as clams' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steam that I've seen before include "Water vapour; impetus" , "Water-vapour" , "Source of energy" , "Energy (colloq.)" , "Momentum - boiled water" .)