Formerly not drinking, wanderer inside becoming outgoing (9)
I believe the answer is:
extrovert
'outgoing' is the definition.
(I've seen this before)
'formerly not drinking wanderer inside' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'not drinking' becomes 'tt' (short for teetotal).
'wanderer' becomes 'rover' (synonyms).
'inside' means one lot of letters goes inside another.
'tt' enclosing 'rover' is 'trovert'.
'ex'+'trovert'='EXTROVERT'
'becoming' is the link.
(Other definitions for extrovert that I've seen before include "party animal" , "Lively and outgoing person" , "One who is interested more in outside things than the inner self" , "A lively sociable person, always up front" , "Person who is more interested in the world than the inner self" .)