A nice mild drunk could be thus (9)
I believe the answer is:
medicinal
'could be thus' is the definition.
I can't judge whether this definition defines the answer.
'a nice mild drunk' is the wordplay.
'drunk' indicates an anagram.
'a'+'nice'+'mild'='anicemild'
'anicemild' anagrammed gives 'MEDICINAL'.
(Other definitions for medicinal that I've seen before include "of health-giving properties" , "With therapeutic properties" , "Lily 15's invention" , "analeptic" , "Good for you!" .)