Working, and ready for attack (5)
I believe the answer is:
onset
'attack' is the definition.
('onset' can be a synonym of 'attack')
'working and ready' is the wordplay.
'working' becomes 'on'.
'and' says to put letters next to each other.
'ready' becomes 'set' ('set' can be similar in meaning to 'ready').
'on'+'set'='ONSET'
'for' is the link.
(Other definitions for onset that I've seen before include "Start - notes (anag)" , "starting?" , "Start - opening moments" , "Initial stage; attack" , "Beginning (of disease etc)" .)