Message from caller failing to entertain old man, we hear (9)
I believe the answer is:
voicemail
'message from caller' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'failing to entertain old man we hear' is the wordplay.
'failing' becomes 'vice' (vice can mean a moral failing).
'to entertain' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'man' becomes 'm' (M is an abbreviation for male or masculine).
'we hear' becomes 'ail' (I can't explain this - if you can you should give a lot more credence to this answer).
'vice' enclosing 'o' is 'voice'.
'voice'+'m'+'ail'='VOICEMAIL'
Can you help me to learn more?
(Another definition for voicemail that I've seen is " What's reached, when someone can't be reached!".)