Models changed rarely (6)
I believe the answer is:
seldom
'rarely' is the definition.
('seldom' can be a synonym of 'rarely')
'models changed' is the wordplay.
'changed' indicates anagramming the letters.
'models' with letters rearranged gives 'SELDOM'.
(Other definitions for seldom that I've seen before include "on rare occasions" , "Only occasionally" , "Infrequent" , "Not very often, rarely" , "Not 21" .)