Set off with scaffolder, behind time (7)
I believe the answer is:
trigger
'set off' is the definition.
('trigger' can be a synonym of 'set off')
'scaffolder behind time' is the wordplay.
'scaffolder' becomes 'rigger' (I've seen this before).
'behind' means one lot of letters go next to another.
'time' becomes 't'.
'rigger' put after 't' is 'TRIGGER'.
'with' acts as a link.
(Other definitions for trigger that I've seen before include "Activating device" , "Releasing device" , "Set in motion - say gunfire" , "Component of gun" , "Start" .)