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' says to put letters next to each other.
'chum' becomes 'mate' (I've seen this in other clues).
'first'+'mate'='FIRST MATE'
'he's an' is the link.
This may not be right. Some or all of it may be part of 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" .)