Old adversary fairly lacking in it (7)
I believe the answer is:
antique
'old' is the definition.
(similar in meaning)
'adversary fairly lacking in it' is the wordplay.
'adversary' becomes 'anti' (I've seen this before).
'fairly' becomes 'quite' (I've seen this before).
'lacking in' indicates named letters should be taken away.
'quite' with 'it' taken away is 'que'.
'anti'+'que'='ANTIQUE'
(Other definitions for antique that I've seen before include "Old item of furniture" , "Archaic" , "of great age" , "Vintage" , "Item from an earlier time" .)