Poly ending (5)
I believe the answer is:
ester
'poly ending' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ester that I've seen before include "radical" , "Chemical compound of acid and alcohol, found in trees" , "Reset (anag.)" , "Compound; trees (anag.)" , "Organic compound, rhymes with court joker" .)