A useful quality when hardened (5)
I believe the answer is:
asset
'a useful quality' is the definition.
(I've seen this before)
'when hardened' is the wordplay.
'when' becomes 'as'.
'hardened' becomes 'set' ('set' can be a synonym of 'hardened').
'as'+'set'='ASSET'
(Other definitions for asset that I've seen before include "Advantage, strength" , "It's an advantage to declare" , "Something useful" , "Holding" , "Valued item" .)