Sober? I assume half of bottle drunk (10)
I believe the answer is:
abstemious
'sober?' is the definition.
(thesaurus)
'i assume half of bottle drunk' is the wordplay.
'half of' indicates taking half.
'drunk' is an anagram indicator.
'bottle' cut in half is 'bot'.
'i'+'assume'+'bot'='iassumebot'
'iassumebot' is an anagram of 'ABSTEMIOUS'.
(Other definitions for abstemious that I've seen before include "refraining from excess" , "Indulging only moderately (in food/drink)" , "I mess about (anag.)" , "Self-denying" , "Drinking little" .)