Just over half of the arboretum was cut back leaving them unable to speak (4)

I believe the answer is:

mute

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'unable to speak' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for mute that I've seen before include "Silent - dumb" , "Silent, refraining from speaking" , "Not pronounced" , "Speechless, soundless" , "Silencer" .)

I've seen this clue in The Irish Times.

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