Plant sap - a sugar substitute (9)
I believe the answer is:
asparagus
'plant' is the definition.
(asparagus is a kind of plant)
'sap a sugar substitute' is the wordplay.
'substitute' indicates anagramming the letters.
'sap'+'a'+'sugar'='sapasugar'
'sapasugar' is an anagram of 'ASPARAGUS'.
(Other definitions for asparagus that I've seen before include "Spears or shoots" , "Succulent edible shoots" , "The Theatre Cat (T. S. Eliot); an edible plant" , "Green shoots, cooked and eaten as a vegetable" , "may be seen on an allotment" .)