Spilling over drink is a disgusting matter (3)
I believe the answer is:
pus
'disgusting matter' is the definition.
'pus' can be an answer for 'matter' (pus is a kind of matter). I am not certain of the 'disgusting' bit.
'spilling over drink' is the wordplay.
'spilling over' shows that the letters should be reversed in order (I've seen 'spilt over' mean this).
'drink' becomes 'sup' (I've seen this before).
'sup' backwards is 'PUS'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for pus that I've seen before include "Fluid in infected tissue" , "Fluid formed by suppuration" , "Liquid produced in infected tissue" , "Material in wounds" , "Yellowish discharge" .)