Retains form that’s not as nice (7)
I believe the answer is:
nastier
'not as nice' is the definition.
('nasty' can be the opposite of 'nice' so perhaps 'nastier' is reasonable for 'not as nice')
'retains form' is the wordplay.
'form' indicates an anagram (a different form of the same letters).
'retains' with letters rearranged gives 'NASTIER'.
'that's' is the link.
(Other definitions for nastier that I've seen before include "More unpleasant or distasteful" , "becomes more unpleasant" , "Less nice" , "More horrid" , "Retsina (anag) -- not so nice" .)