It's better to be thinner (5)
I believe the answer is:
finer
I believe this clue is a double definition.
'it's better' is the first definition.
(I've seen this in another clue)
'be thinner' is the second definition.
('fine' can be similar in meaning to 'thin')
'to' acts as a link.
(Other definitions for finer that I've seen before include "Of higher quality" , "More gossamer or splendid" , "Better; more slender" , "Better quality" , "More thin, excellent" .)