Chemical relative (6)
I believe the answer is:
isomer
'chemical relative' 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 "One of two compounds differing in arrangement of the same atoms" , "Compound sharing a formula but having its own atomic arrangement" , "Compound of identical atoms to another but in differing arrangement" , "Chemical compound - I'm Eros (anag)" , "Compound with same formula as another but not atom arrangement" .)