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