Are they said to be seaside trees? (7)

I believe the answer is:

beeches

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'trees?' is the definition.
(beech is a kind of tree)

'are they said to be seaside' is the wordplay.
'are they said' indicates a 'sounds like' (homophone) clue.
'seaside' becomes 'ches' (I can't explain this - if you can you should give a lot more credence to this answer).
'be' sounds like 'bee'.
'bee'+'ches'='BEECHES'

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I've seen this clue in the Evening Standard.

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