Invalid turned firearm on a Conservative (8)
I believe the answer is:
nugatory
'invalid' is the definition.
(similar in meaning)
'turned firearm on a conservative' is the wordplay.
'turned' is a reversal indicator.
'firearm' becomes 'gun' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'gun' in reverse letter order is 'nug'.
'nug'+'a'+'tory'='NUGATORY'
(Other definitions for nugatory that I've seen before include "Worthless - futile" , "useless" , "Triflingly unimportant" , "Worthless or of no real value" , "Futile, of no value" .)