As an officer, he has his own chair (5)
I believe the answer is:
bosun
'his own chair' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'as an officer he' is the wordplay.
I cannot quite see how this works, but
'he' could be 'bo' and 'bo' is found within the answer.
The remaining letters 'sun' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'has' is the link.
Can you help me to learn more?
(Other definitions for bosun that I've seen before include "A dimunitive boatswain" , "A diminutive boatswain" , "A short boatswain" , "An abbreviated boatswain" , "Naval officer" .)