Perfect being lighter? Not so (9)
I believe the answer is:
matchless
'perfect' is the definition.
(thesaurus)
'lighter? not so' is the wordplay.
'lighter?' becomes 'match' (match is a kind of lighter**).
'not so' becomes 'less' (I've seen this before).
'match'+'less'='MATCHLESS'
'being' is the link.
(Other definitions for matchless that I've seen before include "Unequalled" , "Without peer, nonpariel" , "Perfect" , "Having no equal" , "team without a fixture" .)