Massive drunk rooming with American (9)
I believe the answer is:
ginormous
'massive' is the definition.
(I've seen this in another clue)
'drunk rooming with american' is the wordplay.
'drunk' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'american' becomes 'us' (abbreviation for United States).
'rooming' anagrammed gives 'ginormo'.
'ginormo'+'us'='GINORMOUS'
(Other definitions for ginormous that I've seen before include "Huge" , "Titanic" , "jumbo" , "Big (slang)" .)