Least fresh beer served in street twice (7)
I believe the answer is:
Here is my best explanation:
'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 subsidiary indication.
'beer' becomes 'ale' (I've seen this before).
'served in' is an insertion indicator.
'street' becomes 'st'.
'twice' says to double the letters.
'st' doubled is 'stst'.
'ale' inserted into 'stst' is 'stalest'.
(Other definitions for stalest that I've seen before include "Least original" and "Most unoriginal".)