Rarely failing models (6)
I believe the answer is:
seldom
'rarely' is the definition.
(I know that rarely can be written as seldom)
'failing models' is the wordplay.
'failing' indicates an anagram (a failed spelling of the word).
'models' is an anagram of 'SELDOM'.
(Other definitions for seldom that I've seen before include "Only occasionally" , "Not often, infrequently" , "Once in a blue moon" , "Not very often, rarely" , "Alluvium" .)