Small size (4)
I believe the answer is:
mini
'small size' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mini that I've seen before include "vehicle" , "Once fashionable skirt" , "Car; short dress" , "Iconic economy car (introduced in 1959)" , "Small (of car, skirt, disk,bar or series)" .)