Diet, terrible at first for army unit (8)
I believe the answer is:
regiment
'army unit' is the definition.
(regiment is a kind of army unit)
'diet terrible at first' is the wordplay.
'diet' becomes 'regimen' (I've seen this before).
'terrible at first' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'regimen'+'t'='REGIMENT'
'for' is the link.
Can you help me to learn more?
(Other definitions for regiment that I've seen before include "Military group" , "Organise; army unit" , "Brigade" , "Section of an army" , "Metering (anag) - army unit" .)