Times up: put a single on for a musical interval (8)
I believe the answer is:
semitone
'for a musical interval' is the definition.
(semitone is a kind of musical interval)
'times up put a single' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'put' says to put letters next to each other.
'a single' becomes 'one' (a single thing - only one).
'times' backwards is 'semit'.
'semit'+'one'='SEMITONE'
'on' acts as a link.
(Other definitions for semitone that I've seen before include "One twelfth of an octave" , "Small musical interval" , "Small interval between notes" , "100 cents" , "Short musical interval" .)