Musical instruments spoil second birthday on Monday. Party finished early (8)
I believe the answer is:
marimbas
'musical instruments' is the definition.
(marimba is a kind of musical instrument)
'spoil second birthday on monday party finished early' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'second birthday' becomes 'i' (2nd letter of 'birthday').
'on monday' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'party' becomes 'bash' (term for a party or celebration).
'finished early' means to remove the last letter (the word finishes earlier than normal).
'bash' with its last letter removed is 'bas'.
'mar'+'i'+'m'+'bas'='MARIMBAS'
Can you help me to learn more?
(Another definition for marimbas that I've seen is " Percussion instruments played with small mallets".)