All right - article inserted in tree (3)
I believe the answer is:
oak
'tree' is the definition.
(oak is a type of tree)
'all right article inserted' is the wordplay.
'all right' becomes 'ok' (synonyms).
'article' becomes 'a' (the indefinite article in English).
'inserted' indicates putting letters inside.
'ok' enclosing 'a' is 'OAK'.
'in' is the link.
(Other definitions for oak that I've seen before include "Acorn-bearing tree" , "The mighty tree" , "door at Oxford" , "Large tree" , "Its seed is the acorn" .)