Old man's gone with raincoat undone (12)
I believe the answer is:
octogenarian
'old man's' is the definition.
(thesaurus)
'gone with raincoat undone' is the wordplay.
'with' means one lot of letters go next to another.
'undone' is an anagram indicator.
'gone' put after 'raincoat' is 'raincoatgone'.
'raincoatgone' with letters rearranged gives 'OCTOGENARIAN'.
(Other definitions for octogenarian that I've seen before include "One born nineteen thirteen - nineteen twenty two" , "Augustus John in last years" , "One in their eighties" , "Someone aged between 80 and 89" , "One aged 80 to 89" .)