Basic fault -- and men must change (11)
I believe the answer is:
fundamental
'basic' is the definition.
('fundamental' can be similar in meaning to 'basic')
'fault and men must change' is the wordplay.
'must change' indicates anagramming the letters (I've seen this in other clues).
'fault'+'and'+'men'='faultandmen'
'faultandmen' is an anagram of 'FUNDAMENTAL'.
(Other definitions for fundamental that I've seen before include "Basic" , "Of central importance" , "Primary" , "key" .)