Tree a woman planted outside new councillor’s office (10)

'councillor's office' is the definition.
(the office or duty of an alderman)

'tree a woman planted outside new' is the wordplay.
'tree' becomes 'alder' (alder is a kind of tree).
'a woman' becomes 'mary' ().
'planted outside' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'mary' enclosing 'n' is 'manry'.
'alder'+'manry'='ALDERMANRY'