Crossword Genius

Prime tree replaced in boundary (9)

Ross

I believe the answer is:

perimeter

'boundary' is the definition.
(I know that perimeter is a type of boundary)

'prime tree replaced' is the wordplay.
'replaced' indicates an anagram.
'prime'+'tree'='primetree'
'primetree' is an anagram of 'PERIMETER'.

'in' is the link.

(Other definitions for perimeter that I've seen before include "Continuous boundary" , "Prime tree (anag.)" , "Border" , "Bound" , "bounds" .)

I've seen this clue in The Times.
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