Firing? GI in on it, surprisingly (8)
I believe the answer is:
ignition
'firing?' is the definition.
('ignition' can be a synonym of 'firing')
'gi in on it surprisingly' is the wordplay.
'surprisingly' is an anagram indicator.
'gi'+'in'+'on'+'it'='giinonit'
'giinonit' is an anagram of 'IGNITION'.
(Other definitions for ignition that I've seen before include "Act of lighting burner" , "Car engine starter" , "Point at which combustion occurs" , "Internal-combustion engine's firing system" , "Beginning of combustion" .)