Some relief if there's a musical interval (5)
I believe the answer is:
fifth
'a musical interval' is the definition.
(fifth is a kind of musical interval)
'some relief if there's' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'FIFTH' is hidden within 'relief if theres'.
(Other definitions for fifth that I've seen before include "US amendment guaranteeing the right to silence" , "- Column is a group working for the enemy" , "20 per cent" , "Next after fourth" , "Musical interval" .)