Like tiny particles of mosaic, but different (9)
I believe the answer is:
subatomic
'like tiny particles' is the definition.
(like subatomic particles)
'mosaic but different' is the wordplay.
'different' indicates an anagram.
'mosaic'+'but'='mosaicbut'
'mosaicbut' anagrammed gives 'SUBATOMIC'.
'of' acts as a link.
(Other definitions for subatomic that I've seen before include "Of smaller than elementary particles" , "Smaller than a basic particle of matter" , "Like 9 across" .)