Diet for most of an army unit (7)
I believe the answer is:
regimen
'diet' is the definition.
(I've seen this before)
'most of an army unit' is the wordplay.
'most of' means to remove the last letter.
'an army unit' becomes 'regiment' (regiment is a kind of army unit).
'regiment' with its final letter taken off is 'REGIMEN'.
'for' is the link.
(Other definitions for regimen that I've seen before include "Therapeutic course of treatment" , "Government - course of treatment" , "Merge in (anag.)" , "Systematic course of treatment, diet, etc" , "System of diet etc" .)