Short-vowel mark (5)
I believe the answer is:
breve
'short-vowel mark' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for breve that I've seen before include "Long musical note" , "Note equivalent to four minims" , "Note with the time value of eight crotchets" , "Rarely used musical note" , "(In music) long note" .)