Models work, but not often (6)
I believe the answer is:
seldom
'not often' is the definition.
('seldom' is the opposite to 'often')
'models work' is the wordplay.
'work' is an anagram indicator.
'models' with letters rearranged gives 'SELDOM'.
'but' is the link.
(Other definitions for seldom that I've seen before include "Once in a blue moon" , "Only occasionally" , "Infrequent" , "Alluvium" , "Models (anag)" .)