Chemical unions of two or more elements (9)
I believe the answer is:
compounds
'chemical unions of two or more elements' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for compounds that I've seen before include "Chemical unions of two or more elements" , "Wholes formed from two or more parts" , "settles a debt" , "Mixtures of chemical elements" , "Enclosures of residences and other buildings" .)