Turning over weapon to a Conservative is useless (8)
I believe the answer is:
nugatory
'useless' is the definition.
(I've seen this before)
'turning over weapon to a conservative' is the wordplay.
'turning over' is a reversal indicator.
'weapon' becomes 'gun' (gun is a kind of weapon).
'to' is a charade indicator (letters next to each other).
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'gun' back-to-front is 'nug'.
'nug'+'a'+'tory'='NUGATORY'
'is' is the link.
(Other definitions for nugatory that I've seen before include "Worthless - trifling" , "Insignificant or trivial" , "of little worth" , "Young art (anag) -- worthless" , "Worthless or of no real value" .)