Unusual for a clef to get lost (5,3)
I believe the answer is:
clear off
'get lost' is the definition.
(both can mean go away)
'unusual for a clef' is the wordplay.
'unusual' indicates an anagram.
'for'+'a'+'clef'='foraclef'
'foraclef' is an anagram of 'CLEAR OFF'.
'to' is the link.
(Other definitions for clear off that I've seen before include "Get lost!" , "Leave" , "Begone (colloq.)" , "Go away - transparently" , "Run along!" .)