Small mouse heard? (4)
I believe the answer is:
mini
'small' is the definition.
(short for miniature)
'mouse heard?' is the wordplay.
'mouse' becomes 'Minnie' (Minnie Mouse, girlfriend of Mickey).
'heard?' shows a homophone (sound like).
'minnie' sounds like 'MINI'.
(Other definitions for mini that I've seen before include "Once fashionable skirt" , "type of garment" , "Little - car" , "Small (of car, skirt, disk,bar or series)" , "Iconic economy car (introduced in 1959)" .)