Chemical compound (5)
(Other definitions for ester that I've seen before include "Fat" , "Reset (anag.)" , "radical" , "Chemical compound of acid and alcohol, found in trees" , "Organic chemical compound" .)
(Other definitions for oxide that I've seen before include "Compound of two elements, one with atomic number 8" , "Eg, rust" , "Chemical compound" , "Compound such as rust" , "Rust, for instance" .)
'chemical compound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for isomer that I've seen before include "Compound sharing a formula but having its own atomic arrangement" , "Atomic nucleus; is more (anag.)" , "alternative arrangement of small parts" , "Compound with same formula as another but not atom arrangement" , "Compound with same formula but not structure" .)