Gun that Ruby turned on lookalike (9)
I believe the answer is:
derringer
'gun' is the definition.
(I know that Derringer is a type of pistol)
'ruby turned on lookalike' is the wordplay.
'ruby' becomes 'red' (synonyms).
'turned' says the letters should be written backwards.
'on' means one lot of letters go next to another.
'lookalike' becomes 'ringer' (I've seen this in another clue).
'red' written backwards gives 'der'.
'der'+'ringer'='DERRINGER'
'that' is the link.
(Other definitions for derringer that I've seen before include "Small pistol used by gamblers in western films" , "Weapon" , "Short-barrelled American pistol" , "Pocket pistol" .)