Ratbag to swear it's late? (7)
I believe the answer is:
cadaver
'it's late?' is the definition.
I can't judge whether this defines the answer.
'ratbag to swear' is the wordplay.
'ratbag' becomes 'cad' (I am not sure about this - if you are sure you should believe this answer much more).
'to swear' becomes 'aver' (synonyms).
'cad'+'aver'='CADAVER'
Can you help me to learn more?
(Other definitions for cadaver that I've seen before include "Human corpse" , "Stiff" , "Remains of a human" , "A kind of corpse" , "Dead body, corpse" .)