Do set fire to the tree (7)
I believe the answer is:
conifer
'the tree' is the definition.
(conifer is a kind of tree)
'do set fire' is the wordplay.
'do' becomes 'con'.
'set' indicates an anagram (instruction to set the letters).
'fire' is an anagram of 'ifer'.
'con'+'ifer'='CONIFER'
'to' is the link.
(Other definitions for conifer that I've seen before include "Tree such as pine, yew etc" , "Fir or pine, for example" , "In force (anag.)" , "Type of tree" , "Evergreen tree" .)