Tress guard who helps lower boats in canal? (4-6)

I believe the answer is:

lock-keeper

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'helps lower boats in canal?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'tress guard' is the wordplay.
'tress' becomes 'lock' (I've seen this before).
'guard' becomes 'keeper' (someone keeping or guarding something).
'lock'+'keeper'='LOCK-KEEPER'

'who' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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