Laments day-old fails (8)
I believe the answer is:
Here is my best explanation:
'laments' is the definition.
(a monody is a poem written in mourning)
'day-old fails' is the wordplay.
'day' becomes 'mon'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fails' becomes 'dies' ('die' can be a synonym of 'fail').
(Another definition for monodies that I've seen is "solos?".)