Model's working on rare occasions (6)
I believe the answer is:
seldom
'on rare occasions' is the definition.
(I've seen this in another clue)
'model's working' is the wordplay.
'working' is an anagram indicator.
'models' is an anagram of 'SELDOM'.
(Other definitions for seldom that I've seen before include "Models (anag)" , "Once in a blue moon" , "Not often, infrequently" , "Only occasionally" , "Not 21" .)