Adult, one after two pints briefly confronting American stars (8)
I believe the answer is:
aquarius
'stars' is the definition.
'adult one after two pints briefly confronting american' is the wordplay.
'adult' becomes 'a'.
'one after' means one lot of letters go next to another.
'two pints' becomes 'quart' (I've seen this before).
'briefly' means to remove the last letter.
'confronting american' becomes 'ius' (I can't justify this - if you can you should believe this answer much more).
'quart' with its last letter taken away is 'quar'.
'a'+'quar'+'ius'='AQUARIUS'
(Other definitions for aquarius that I've seen before include "Sign of the zodiac, the water-bearer" , "Star sign" , "A 23 ac" , "Sign of the water-carriers" , "Sign of the zodiac, the water-bearers" .)