Democratic Unionist in a tree (7)
I believe the answer is:
popular
'democratic' is the definition.
(I know that democratic can be written as popular)
'unionist in a tree' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'in' is an insertion indicator.
'a tree' becomes 'poplar' (poplar is a kind of tree).
'u' inserted into 'poplar' is 'POPULAR'.
(Other definitions for popular that I've seen before include "Demotic" , "Pleasing to many" , "In general favour" , "Widely admired" , "Non-technical" .)