Rubbish written about a southern cook (5)
I believe the answer is:
roast
'cook' is the definition.
(I know that roast is a more specific form of cook)
'rubbish written about a southern' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'written about' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'a'+'s'='as'
'rot' placed around 'as' is 'ROAST'.
(Other definitions for roast that I've seen before include "Have a go at" , "Cooked joint" , "Reprimand severely" , "Oven-cooked joint" , "Cook (meat) over a fire" .)