Unit gets diet on time (8)
I believe the answer is:
regiment
'unit' is the definition.
(regiment is a kind of unit)
'diet on time' is the wordplay.
'diet' becomes 'regimen' (a diet is a certain eating regimen).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'regimen'+'t'='REGIMENT'
'gets' is the link.
(Other definitions for regiment that I've seen before include "Army grouping" , "Large army unit" , "a whole lot of them" , "Section of an army" , "The Grenadier Guards, for example" .)