Rule diet incomplete (6)
I believe the answer is:
regime
'rule' is the definition.
(thesaurus)
'diet incomplete' is the wordplay.
'diet' becomes 'regimen' (a diet is a certain eating regimen).
'incomplete' means to remove the last letter.
'regimen' with its final letter taken off is 'REGIME'.
(Other definitions for regime that I've seen before include "Government; ordered way of doing something" , "A governing administration" , "Ruling system" , "co-ordinated programme" , "Plan for therapy or government in power" .)