Least fresh beer served in street twice (7)
I believe the answer is:
stalest
'least fresh' is the definition.
('stale' is the opposite of 'fresh' so 'stalest' should be a synonym for 'least fresh')
'beer served in street twice' is the wordplay.
'beer' becomes 'ale' (I've seen this before).
'served in' indicates putting letters inside.
'street' becomes 'st'.
'twice' shows that letters should be duplicated.
'st' duplicated is 'stst'.
'ale' going inside 'stst' is 'STALEST'.
(Other definitions for stalest that I've seen before include "Least original" , "Most unoriginal" .)