Warmer attack (4,4)
I believe the answer is:
open fire
This clue is a double definition.
'warmer' is the first definition.
(something which warms)
'attack' is the second definition.
('open fire' is an instruction to attack)
(Other definitions for open fire that I've seen before include "Begin to shoot" , "traditional welcoming feature" , "Begin shooting" , "order for soldiers?" , "Get rounds in" .)