Endless cheating: a case for the doctor? (10)
I believe the answer is:
adulterate
'doctor?' is the definition.
(both can mean to mix with something else)
'endless cheating a case for the' is the wordplay.
'endless' means to remove the last letter.
'cheating' becomes 'adultery' (cheating on a partner).
'case for' says to hollow out the word (remove centre letters) (the letters which encase the others are used).
'the' with its centre removed is 'te'.
'adultery' with its last letter taken away is 'adulter'.
'adulter'+'a'+'te'='ADULTERATE'
(Other definitions for adulterate that I've seen before include "Reduce purity" , "Debase by mixing with something inferior" , "Make impure by addition" , "Doctor" , "add inferior material?" .)