Fantastic mosaic - but very small (9)
I believe the answer is:
subatomic
'very small' is the definition.
'fantastic mosaic but' is the wordplay.
'fantastic' indicates anagramming the letters.
'mosaic'+'but'='mosaicbut'
'mosaicbut' anagrammed gives 'SUBATOMIC'.
(Other definitions for subatomic that I've seen before include "Of smaller than elementary particles" , "Like 9 across" , "Smaller than a basic particle of matter" .)