Gooey substance produced by small tree (5)
I believe the answer is:
slime
'gooey substance' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'by small tree' is the wordplay.
'by' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tree' becomes 'lime' (type of tree).
's' next to 'lime' is 'SLIME'.
'produced' acts as a link.
Can you help me to learn more?
(Other definitions for slime that I've seen before include "Unpleasantly moist and slippery substance" , "Gooey seeping stuff" , "Goo" , "Gunk" , "Slippery mud; slug trail" .)