To start with, chum, he's an officer (5,4)
I believe the answer is:
first mate
'officer' is the definition.
(first mate is a kind of officer)
'to start with chum' is the wordplay.
'to start' becomes 'first' (synonyms).
'with' means one lot of letters go next to another.
'chum' becomes 'mate' (I've seen this before).
'first'+'mate'='FIRST MATE'
'he's an' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for first mate that I've seen before include "Second in command on ship, like primary spouse" , "Ship's officer" , "officer on ship" , "Second in command on a merchant ship" , "Eve" .)