Can, missing round, get sober (9)
I believe the answer is:
abstinent
'sober' is the definition.
('abstinent' can be similar in meaning to 'sober')
'can missing round' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'missing' becomes 'absent' ('absent' can be similar in meaning to 'missing'**).
'round' means one lot of letters goes inside another.
'tin' going inside 'absent' is 'ABSTINENT'.
'get' is the link.
(Other definitions for abstinent that I've seen before include "Avoiding alcohol" , "self-control" , "Refraining from drinking alcohol" , "Not taking drink" , "On the wagon" .)