Start working hard and fast (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'working hard and fast' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'hard and fast' becomes 'set' (I've seen this in another clue).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Beginning; violent attack" , "Start - notes (anag)" , "Advent" , "Start - opening moments" , "Assault; start" .)