Diet soldiers left unfinished (7)
I believe the answer is:
regimen
'diet' is the definition.
(a diet is a certain eating regimen)
'soldiers left unfinished' is the wordplay.
'soldiers' becomes 'regiment'.
'left unfinished' means to remove the last letter.
'regiment' with its final letter removed is 'REGIMEN'.
(Other definitions for regimen that I've seen before include "Method of working" , "Government - course of treatment" , "Merge in (anag.)" , "(In medicine) a systematic plan for therapy" , "Recommended course" .)