Veteran's gone out in shabby raincoat (12)
I believe the answer is:
octogenarian
'veteran's' is the definition.
(thesaurus)
'gone out in shabby raincoat' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'in' indicates putting letters inside.
'shabby' indicates an anagram.
'gone' with letters rearranged gives 'ogen'.
'raincoat' is an anagram of 'octarian'.
'ogen' put into 'octarian' is 'OCTOGENARIAN'.
(Other definitions for octogenarian that I've seen before include "One in their eighties" , "Someone aged between 80 and 89" , "Augustus John in last years" , "One born nineteen thirteen - nineteen twenty two" , "Person over 80" .)