Unexpectedly assign old slide (9)
I believe the answer is:
glissando
'slide' is the definition.
(sliding musical effect)
'unexpectedly assign old' is the wordplay.
'unexpectedly' indicates an anagram.
'assign'+'old'='assignold'
'assignold' is an anagram of 'GLISSANDO'.
(Other definitions for glissando that I've seen before include "Re-interpreting this musical direction" , "trombone slide, for example" , "Effect produced by playing a series of notes very rapidly" , "Sliding effect in moving from one musical note to another" , "Slide between two notes" .)