Frequently, a cracked note is out of key (5)
I believe the answer is:
often
'frequently' is the definition.
(synonyms)
'a cracked note is out of key' is the wordplay.
'a cracked' indicates anagramming the letters.
'is out of' indicates putting letters inside (some letters on the outside of others).
'key' becomes 'f' (musical key).
'note' with letters rearranged gives 'oten'.
'oten' enclosing 'f' is 'OFTEN'.
(Other definitions for often that I've seen before include "Perhaps daily" , "Frequent" , "on many occasions" , "In many cases" , "Frequently, not seldom" .)