Merchant vessel perhaps essential for spying (10)

I believe the answer is:

tradecraft

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'essential for spying' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'merchant vessel' is the wordplay.
'merchant' becomes 'trade' (I can't explain this - if you can you should give a lot more credence to this answer).
'vessel' becomes 'craft' (synonyms).
'trade'+'craft'='TRADECRAFT'

'perhaps' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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