Rue absence, occasionally missing old instrument (5)
I believe the answer is:
rebec
'old instrument' is the definition.
(I've seen this before)
'rue absence occasionally missing' is the wordplay.
'occasionally missing' indicates alternate letters (letters removed at regular intervals or occasions).
'rue'+'absence'='rueabsence'
The alternating letters of 'rueabsence' are 'REBEC'.
(Other definitions for rebec that I've seen before include "Mediaeval stringed instrument" , "Medieval bowed instrument" , "Old stringed instrument" .)