Tiny particle, or two fragments of iron (3)
I believe the answer is:
ion
'tiny particle' is the definition.
(I've seen this in another clue)
'two fragments of iron' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'or' acts as a link.
Can you help me to learn more?
(Other definitions for ion that I've seen before include "Subatomic particle" , "Eletrically charged particle" , "Electrically charged atom or group of atoms" , "Electrically charged particle" , "Electrically-charged particle" .)