Like some senior councillors raving under tree (10)
I believe the answer is:
aldermanic
'like some senior' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.
'councillors raving under tree' is the wordplay.
'councillors raving' becomes 'manic' (I can't justify this - if you can you should give a lot more credence to this answer).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'tree' becomes 'alder' (alder is a kind of tree).
'manic' after 'alder' is 'ALDERMANIC'.
Can you help me to learn more?
(Other definitions for aldermanic that I've seen before include "Old councillor's" , "of civic dignitaries" , "civic dignitary thus described?" , "Like council member" .)