Half note (5)
I believe the answer is:
minim
'half note' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for minim that I've seen before include "Note with value of half a semibreve" , "It's a downward stroke" , "Half-note" , "Two crotchets in one" , "A note in music, half as long as a semi-breve- it's a palindrome" .)