The pale mixture that's left in the glass (7)
I believe the answer is:
heeltap
'left in the glass' is the definition.
The definition suggests a singular noun which matches the answer.
'the pale mixture' is the wordplay.
'mixture' indicates anagramming the letters.
'the'+'pale'='thepale'
'thepale' is an anagram of 'HEELTAP'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for heeltap that I've seen before include "Last drop in glass" , "Drop left in glass" , "Shoe-layer material; dregs in glass" , "Layer of leather at the back of a shoe" , "little left to drink" .)