It's an insect, child, on the tree (5)
I believe the answer is:
beech
'tree' is the definition.
(beech tree is a kind of tree)
'it's an insect child on' is the wordplay.
'it's an insect' becomes 'bee' (bee is a type of insect).
'child' becomes 'ch' (abbreviation for child).
'on' says to put letters next to each other.
'bee'+'ch'='BEECH'
'the' is the link.
(Other definitions for beech that I've seen before include "Tree (with green or copper leaves)" , "Wood" , "It has branches" , "Tree with bitter 'nuts'" , "for mast?" .)