One of three in a water molecule (4)
I believe the answer is:
atom
'one of three in a water molecule' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atom that I've seen before include "Elementary particle" , "Bit of make-up" , "A mighty particle, can be explosive" , "Smallest amount" , "Smallest component of an element" .)