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Initial friend on board (5,4)

Ross

I believe the answer is:

first mate

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'board' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'initial friend on' is the wordplay.
'initial' becomes 'first' (similar in meaning).
'friend' becomes 'mate' (mate is a kind of friend).
'on' says to put letters next to each other.
'first'+'mate'='FIRST MATE'

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" , "sailor?" , "Second in command on a merchant ship" , "officer on ship" , "Eve" .)

I've seen this clue in The Mirror.
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