Smaller than a tiny metal box (3)
I believe the answer is:
tin
'box' is the definition.
(I've seen this in other clues)
'smaller than a tiny metal' is the wordplay.
I cannot quite understand how this works, but
'metal' could be 'in' (In is an example) and 'in' is found in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tin that I've seen before include "Metal element" , "Atomic number 50" , "Silvery white metal" , "Silvery metallic element" , "Ready" .)