Part-time models earn more (4)
I believe the answer is:
else
'more' is the definition.
('else' can be a synonym of 'more')
'part-time models earn' is the wordplay.
'part' says the answer is hidden in the clue.
'ELSE' is hidden in the letters of 'time models earn'.
(Other definitions for else that I've seen before include "In addition; different" , "Otherwise; different from" , "As an alternative" , "Other than (what is under consideration)" , "Besides; instead" .)