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' says the letters should be written in reverse.
'firearm' becomes 'gun' (I've seen this in another clue).
'on' says to put letters next to each other.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'gun' back-to-front 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" .)