In the army, in short, he wouldn't have one left! (7)
I believe the answer is:
colonel
'in the army in short he wouldn't have one left' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he wouldn't have one left' is the wordplay.
'he wouldn't' becomes 'col' (I can't explain this - if you can you should believe this answer much more).
'have' means one lot of letters go next to another.
'left' becomes 'l' (common abbreviation).
'col'+'one'+'l'='COLONEL'
Can you help me to learn more?
(Other definitions for colonel that I've seen before include "Rank (below brigadier)" , "No cello (anag.)" , "Army three downn" , "Commissioned officer" , "Army rank" .)