1 in a tree-lined avenue? (5)
I believe the answer is:
first
'1' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'tree-lined avenue?' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'lined avenue?' becomes 'st' (I can't explain this - if you can you should believe this answer much more).
'fir'+'st'='FIRST'
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for first that I've seen before include "Initial" , "Most important" , "best" , "Prime" , "Preceding all others" .)