Heard you hurried over and called my boss impromptu (10)

'impromptu' is the definition.
(thesaurus)

'heard you hurried over and called my boss' is the wordplay.
'heard' shows a homophone (sound like).
'hurried over' becomes 'nar' (I am not sure about this - if you are sure you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'called' becomes 'rang' (called on the phone).
'my boss' becomes 'ed' (abbreviation for editor, the setter's boss).
'you' is a homophone of 'u'.
'u'+'nar'+'rang'+'ed'='UNARRANGED'

Can you help me to learn more?