Useful thing when fixed (5)
I believe the answer is:
asset
'useful thing' is the definition.
(I've seen this before)
'when fixed' is the wordplay.
'when' becomes 'as'.
'fixed' becomes 'set' ('set' can be a synonym of 'fixed').
'as'+'set'='ASSET'
(Other definitions for asset that I've seen before include "Advantage, strength" , "one liable to be stripped?" , "virtue" , "valuable sort" , "Something valuable to have" .)