Army unit's incomplete diet (7)
I believe the answer is:
regimen
'diet' is the definition.
(I've seen this before)
'army unit's incomplete' is the wordplay.
'army unit' becomes 'regiment' (regiment is a kind of army unit).
'incomplete' means to remove the last letter.
'regiment' with its final letter removed is 'REGIMEN'.
(Other definitions for regimen that I've seen before include "That's what doctor orders" , "Mere gin for a course of treatment" , "Prescribed course of diet, exercise, etc" , "Method of working" , "System of diet etc" .)