The first scaffolding worker set off (7)
I believe the answer is:
trigger
'set off' is the definition.
(synonyms)
'the first scaffolding worker' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'scaffolding worker' becomes 'rigger' (I've seen this before).
't'+'rigger'='TRIGGER'
(Other definitions for trigger that I've seen before include "Start" , "Lever to activate firing mechanism" , "One pulled to fire" , "prompt" , "Part of gun" .)