Prime tree replaced in boundary (9)
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" .)