Trim small trees (7)

I believe the answer is:

spruces

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'trim small trees' is the definition.
'spruces' can be an answer for 'trees' (spruce is a kind of tree). I am unsure of the 'trim small' bit.

'trim small trees' is the wordplay.
'trim' becomes 'spruce' (synonyms).
'small trees' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'spruce'+'s'='SPRUCES'

Can you help me to learn more?

(Another definition for spruces that I've seen is " trees sold recently?".)

I've seen this clue in The Guardian.

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