Introducing terrific heat allows water to become warmer (4)
I believe the answer is:
thaw
'become warmer' is the definition.
(I've seen this in another clue)
'introducing terrific heat allows water' is the wordplay.
'introducing' says to take the initial letters.
The initial letters of 'terrific heat allows water' is 'THAW'.
'to' acts as a link.
(Other definitions for thaw that I've seen before include "detente" , "(Of ice/snow) melt" , "spring weather?" , "snow does eventually" , "Unfreeze, defrost" .)