Models change infrequently (6)
I believe the answer is:
seldom
'infrequently' is the definition.
(I've seen this in another clue)
'models change' is the wordplay.
'change' is an anagram indicator.
'models' anagrammed gives 'SELDOM'.
(Other definitions for seldom that I've seen before include "Alluvium" , "Only occasionally" , "Models (anag)" , "on rare occasions" , "Not 21" .)