Mate in tree, reportedly (4)
I believe the answer is:
pair
'mate' is the definition.
(synonyms)
'tree reportedly' is the wordplay.
'tree' becomes 'pear' (pear is a kind of tree).
'reportedly' indicates a 'sounds like' (homophone) clue.
'pear' sounds like 'PAIR'.
'in' is the link.
(Other definitions for pair that I've seen before include "Put together - two of a kind" , "item" , "Two of a similar type" , "Agreement to avoid division" , "Twins" .)