Like a councillor to be tree-crazy (10)
I believe the answer is:
aldermanic
'like a councillor' is the definition.
(like an alderman)
'tree-crazy' is the wordplay.
'tree' becomes 'alder' (alder tree is a kind of tree).
'crazy' becomes 'manic' (I've seen this before).
'alder'+'manic'='ALDERMANIC'
'to be' acts as a link.
(Other definitions for aldermanic that I've seen before include "Old councillor's" , "of civic dignitaries" , "civic dignitary thus described?" , "Like council member" .)