Awful nonsense – catch up! (6)
I believe the answer is:
rotten
'awful' is the definition.
(thesaurus)
'nonsense catch up' is the wordplay.
'nonsense' becomes 'rot' (I've seen this before).
'catch' becomes 'net' (netting is a kind of catching).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'net' reversed gives 'ten'.
'rot'+'ten'='ROTTEN'
(Other definitions for rotten that I've seen before include "Decayed or despicable" , "Corrupt; very bad" , "Decayed or detestable" , "Something is ... in the state of Denmark (Hamlet)" , "Decayed or corrupt (60" .)