A street to flatten out for a gentle walk (6)
I believe the answer is:
stroll
'gentle walk' is the definition.
(I've seen this before)
'a street to flatten out for a' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'street' becomes 'st' (abbreviation).
'to flatten' becomes 'roll' (rolling is a kind of flattening).
'out' suggests deleting specific letters.
'for a' becomes 'per' ('per' can mean 'for each' person or 'for a' person).
'per'+'st'+'roll'='perstroll'
'perstroll' with 'per' removed is 'STROLL'.
(Other definitions for stroll that I've seen before include "A pleasant and leisurely walk" , "swan" , "A casual or lazy walk" , "walk in a leisurely manner" , "Meander" .)