Hardened against working, for a start (5)
I believe the answer is:
onset
'a start' is the definition.
(onset is a kind of start)
'hardened against working' is the wordplay.
'hardened' becomes 'set' ('set' can be a synonym of 'hardened').
'against' says to put letters next to each other.
'working' becomes 'on'.
'set' after 'on' is 'ONSET'.
'for' is the link.
(Other definitions for onset that I've seen before include "starting?" , "Attack using stone" , "Storming" , "Beginning or early stages" , "Appearance of" .)