A tree is unpopular, but useful partly (7)
I believe the answer is:
arbutus
'a tree' is the definition.
(I've seen this before)
'unpopular but useful partly' is the wordplay.
'partly' says the answer is hidden in the clue.
'ARBUTUS' is hidden in the letters of 'unpopular but useful'.
'is' is the link.
(Other definitions for arbutus that I've seen before include "Strawberry-tree" , "Evergeen" , "Plant" , "But Ursa could become a strawberry tree" , "Evergreen tree or shrub with strawberry-like fruits" .)