Affix a note to the case (7)
I believe the answer is:
attache
'the case' is the definition.
(attache case is a kind of case)
'affix a note' is the wordplay.
'affix' becomes 'attach' (I've seen this before).
'a note' becomes 'e' (musical note).
'attach'+'e'='ATTACHE'
'to' acts as a link.
(Other definitions for attache that I've seen before include "Junior ambassador" , "Man with a mission" , "Briefcase or diplomat" , "Specialist on embassy staff" , "Small rectangular document case" .)