Remarkable guns liar replaced (8)
I believe the answer is:
singular
'remarkable' is the definition.
(I know that remarkable can be written as singular)
'guns liar replaced' is the wordplay.
'replaced' indicates anagramming the letters.
'guns'+'liar'='gunsliar'
'gunsliar' with letters rearranged gives 'SINGULAR'.
(Other definitions for singular that I've seen before include "Remarkable; not plural" , "Extraordinary - conspicuous - sole" , "Odd; unique" , "As sheep may be, but not cattle or horses" , "Unusual - not plural" .)