Be on diet fixed and disciplined (8)
I believe the answer is:
obedient
'disciplined' is the definition.
(both can mean well-behaved)
'be on diet fixed' is the wordplay.
'fixed' indicates an anagram.
'be'+'on'+'diet'='beondiet'
'beondiet' with letters rearranged gives 'OBEDIENT'.
'and' acts as a link.
(Other definitions for obedient that I've seen before include "Willing to comply" , "Like monk" , "co-operative?" , "Ready to submit to authority" , "Submissive, compliant" .)