Little friend of Mickey being heard (4)
I believe the answer is:
mini
'little' is the definition.
(thesaurus)
'friend of mickey being heard' is the wordplay.
'friend of mickey' becomes 'Minnie' (Mickey Mouse and Minnie Mouse).
'being heard' indicates a 'sounds like' (homophone) clue.
'minnie' is a homophone of 'MINI'.
(Other definitions for mini that I've seen before include "Brief" , "(Car etc) of small size" , "Short skirt - small car" , "Car designed by Issigonis" , "Italian Job car?" .)