Even smaller than miniature! (4)
I believe the answer is:
mini
'miniature' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for mini that I've seen before include "type of garment" , "Small (of car, skirt, disk,bar or series)" , "Italian Job car?" , "Baby" , "Little - car" .)