Not even the cat fails to eat what's left in the pot (3,6)
I believe the answer is:
tea leaves
'eat what's left in the pot' is the definition.
I can't tell whether this defines the answer.
'not even the cat fails' is the wordplay.
'not even' means one should take alternating letters (remove even-numbered letters).
'fails' becomes 'leaves' (I am not sure about this - if you are sure you should believe this answer much more).
'the'+'cat'='thecat'
The alternating letters of 'thecat' are 'tea'.
'tea'+'leaves'='TEA-LEAVES'
'to' is the link.
Can you help me to learn more?
(Other definitions for tea leaves that I've seen before include "Magpies" , "Source of drink" , "Thieves" , "found among the dregs" .)