Working? Way to get over bit of ennui for a start! (5)
I believe the answer is:
onset
'for a start' is the definition.
(both can mean the beginning of something)
'working? way to get over bit of ennui' is the wordplay.
'working?' becomes 'on' (on can mean working or activated).
'way' becomes 'st' (abbreviation for street).
'to get over' means one lot of letters goes inside another.
'bit of' says to take the initial letters.
The initial letter of 'ennui' is 'e'.
'st' enclosing 'e' is 'set'.
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Start - opening moments" , "Initial stage; attack" , "Assault; beginning" , "Beginning; military attack" , "Start, beginning" .)