Drink to queen is more brief (7)
I believe the answer is:
shorter
'more brief' is the definition.
('short' can be a synonym of 'brief')
'drink to queen' is the wordplay.
'drink' becomes 'short'.
'to queen' becomes 'er' (abbreviation for Elizabeth Regina).
'short'+'er'='SHORTER'
'is' acts as a link.
(Other definitions for shorter that I've seen before include "More limited in duration" , "With less length" , "Not as long" , "Brusquer" , "Lower in stature" .)