Sack — cause for alarm? (4)
I believe the answer is:
fire
I believe this clue is a double definition.
'sack' is the first definition.
(I know that sack can be written as fire)
'cause for alarm?' is the second definition.
(I've seen this in another clue)
(Other definitions for fire that I've seen before include "Let off" , "Set light to; shoot" , "... or combustion" , "Shoot; sack" , "cut" .)