Fool knocked over most of coffee, a kind of sap (3,6)
I believe the answer is:
gum arabic
'a kind of sap' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe there's an association between them I don't understand?
'fool knocked over most of coffee' is the wordplay.
'fool' becomes 'mug' (synonyms).
'knocked over' is a reversal indicator.
'most of' means to remove the last letter.
'coffee' becomes 'arabica' (type of coffee plant).
'mug' in reverse letter order is 'gum'.
'arabica' with its final letter removed is 'arabic'.
'gum'+'arabic'='GUM ARABIC'
Can you help me to learn more?
(Other definitions for gum arabic that I've seen before include "Exudation used in glues and incense" , "Substance exuded by acacia" , "Ingredient of 16 across" , "glue" .)