Officer in company — with no company left (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'company with no company left' is the wordplay.
'company' becomes 'co' (abbreviation).
'with no company' becomes 'lone' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'left' becomes 'l' (common abbreviation).
'co'+'lone'+'l'='COLONEL'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for colonel that I've seen before include "Bogey perhaps" , "Army three downn" , "Rank (below brigadier)" , "Commissioned officer" , "No cello (anag.)" .)