Some of the worst ale rejected was not as fresh (6)
I believe the answer is:
staler
'not as fresh' is the definition.
('stale' can be the opposite of 'fresh' so perhaps 'staler' is reasonable for 'not as fresh')
'some of the worst ale rejected' is the wordplay.
'some of' indicates a hidden word.
'STALER' is hidden within 'worst ale rejected'.
'was' acts as a link.
(Other definitions for staler that I've seen before include "not so original" , "Alters (anag) -- less fresh" , "Older and lacking freshness" , "Less fresh, more musty" , "More musty or mouldy" .)