I think we should set off fires (4,2)
I believe the answer is:
lets go
'fires' is the definition.
(I've seen this before)
'i think we should set off' is the wordplay.
'i think we should' becomes 'lets' (I've seen this in another clue).
'set off' becomes 'go' (I've seen this in other clues).
'lets'+'go'='LETS GO'
(Other definitions for lets go that I've seen before include "Come on then, we're off!" , "Releases from one's grip" , "Liberates" , "Order to leave" , "fires" .)