Make up, though one's better without (9)
I believe the answer is:
improvise
'make up' is the definition.
(I've seen this before)
'one's better without' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'better' becomes 'improve' ('improve' can be a synonym of 'better'**).
'without' indicates putting letters inside.
'is' going into 'improve' is 'IMPROVISE'.
'though' is the link.
(Other definitions for improvise that I've seen before include "Quickly make changes" , "Use whatever materials are to hand" , "play by ear" , "Spontaneously create" , "ad lib" .)