Not appealing (12)
I believe the answer is:
unattractive
'not appealing' is the definition.
(I've seen this before)
'not appealing' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'appealing' becomes 'attractive' (similar in meaning).
'un'+'attractive'='UNATTRACTIVE'
(Other definitions for unattractive that I've seen before include "Plain" , "Not appealing" , "Repulsive" , "Ill-favoured" .)