Like a very small bit of mosaic, but cast
I believe the answer is:
subatomic
'like a very small bit' is the definition.
'mosaic but cast' is the wordplay.
'cast' is an anagram indicator.
'mosaic'+'but'='mosaicbut'
'mosaicbut' with letters rearranged gives 'SUBATOMIC'.
'of' acts as a link.
(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" .)