Tree fellers formally will be senior citizens (8)
I believe the answer is:
aldermen
'formally will be senior citizens' is the definition.
I can't judge whether this definition defines the answer.
'tree fellers' is the wordplay.
'tree' becomes 'alder' (alder tree is a kind of tree).
'fellers' becomes 'men' (I can't explain this - if you can you should give a lot more credence to this answer).
'alder'+'men'='ALDERMEN'
Can you help me to learn more?
(Other definitions for aldermen that I've seen before include "Civil dignitaries" , "Civic dignitaries" , "former councillors" .)