Discreet warning: back of dump is foul (3-3)
I believe the answer is:
tip-off
'discreet warning' is the definition.
(I've seen this before)
'back of dump is foul' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
'dump' could be 'tip' (as in a rubbish tip) and 'tip' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tip-off that I've seen before include "Confidential hint" , "Warn helpfully" , "Piece of confidential information" , "Give a secret warning" , "Give advance warning or inside information" .)