Start working with firm (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'working with firm' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'with' says to put letters next to each other.
'firm' becomes 'set' (both can mean fixed).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Assault; start" , "Beginning; military attack" , "Start - opening moments" , "Beginning; violent attack" , "Cast stone in attack" .)