South American postman working in Barking again (10)
I believe the answer is:
patagonian
'south american' is the definition.
(I've seen this before)
'postman working in barking again' is the wordplay.
'postman' becomes 'pat' (children's TV character Postman Pat).
'working' becomes 'on' (on can mean working or activated).
'in' means one lot of letters goes inside another.
'barking' indicates anagramming the letters ('barking' can informally mean mad).
'again' anagrammed gives 'agian'.
'on' put inside 'agian' is 'agonian'.
'pat'+'agonian'='PATAGONIAN'
(Other definitions for patagonian that I've seen before include "From southern Argentina and Chile" , "South American" .)