Change of key heard occasionally (5)
I believe the answer is:
alter
'change' is the definition.
(altering is a kind of changing)
'key heard occasionally' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternate letters of 'heard' are 'er'.
'alt'+'er'='ALTER'
'of' is the link.
(Other definitions for alter that I've seen before include "Change, modify" , "Alert to change" , "Become different" , "Metamorphose" , "Bout of extravagant spending" .)