Brief note left incomplete (4)
I believe the answer is:
mini
'brief' is the definition.
(synonyms)
'note left incomplete' is the wordplay.
'note' becomes 'minim' (type of musical note).
'left incomplete' means to remove the last letter.
'minim' with its last letter taken off is 'MINI'.
(Other definitions for mini that I've seen before include "(Car etc) of small size" , "Baby" , "vehicle" , "Cult British car" , "Iconic economy car (introduced in 1959)" .)