Diet? Army unit shedding tons! (7)
I believe the answer is:
regimen
'diet?' is the definition.
(a diet is a certain eating regimen)
'army unit shedding tons' is the wordplay.
'army unit' becomes 'regiment' (regiment is a kind of army unit).
'shedding' suggests deleting specific letters (shed can mean to lose or cast off).
'tons' becomes 't'.
'regiment' with 't' taken away is 'REGIMEN'.
(Other definitions for regimen that I've seen before include "Recommended course" , "Merge in for prescribed way of life" , "Mere gin for a course of treatment" , "Prescribed way of life like diet" , "Systematic course of treatment, diet, etc" .)