A bad apple is so absolutely corrupt (6,2,3,4)
I believe the answer is:
rotten to the core
'a bad apple' is the definition.
(I've seen this before)
'so absolutely corrupt' is the wordplay.
I cannot really understand how this works, but
'corrupt' could be 'rotten' (synonyms) and 'rotten' is present in the answer.
The remaining letters 'tothecore' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Another definition for rotten to the core that I've seen is " A bad apple".)