The rotters gas for about half an hour! (4)
I believe the answer is:
curs
'the rotters gas' is the definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.
'for about half an hour' is the wordplay.
'for' becomes 'cs' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'half an' suggests halving the letters.
'hour' cut in half is 'ur'.
'cs' enclosing 'ur' is 'CURS'.
Can you help me to learn more?
(Other definitions for curs that I've seen before include "Mongrel dogs" , "Scoundrels; dogs" .)