Lacking gravity, that's plain (12)
I believe the answer is:
unattractive
I believe this clue is a double definition.
'lacking gravity' is the first definition.
(gravity attracts things)
'plain' is the second definition.
(not physically attractive)
'that's' acts as a link.
(Other definitions for unattractive that I've seen before include "Ill-favoured" , "Plain" , "Repulsive" , "Not appealing" .)