Eve maybe one on board (5,4)
I believe the answer is:
first mate
'eve' is the definition.
(I've seen this before)
'maybe one on board' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'on' could be 'at' (I've seen this in other clues) and 'at' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for first mate that I've seen before include "Ship's officer" , "Second in command on a merchant ship" , "officer on ship" , "Second in command on ship, like primary spouse" , "sailor?" .)