Unusual three-bar interval (8)
I believe the answer is:
breather
'interval' is the definition.
(breather is a kind of interval)
'unusual three-bar' is the wordplay.
'unusual' indicates an anagram.
'three'+'bar'='threebar'
'threebar' is an anagram of 'BREATHER'.
(Other definitions for breather that I've seen before include "Short rest between bouts of exercise" , "Recovery time" , "An informal break - inhale and exhale" , "A rest - lung?" , "lung or > liver?" .)