Low-down on MoMA perhaps protected by a spoiler (5-6)
I believe the answer is:
party-pooper
'a spoiler' is the definition.
(I've seen this in another clue)
'low-down on moma perhaps protected by' is the wordplay.
'low-down' becomes 'poop' (synonyms).
'on' is an insertion indicator.
'moma' becomes 'parter' (I can't justify this - if you can you should give a lot more credence to this answer).
'perhaps' becomes 'y' (I've seen this in other clues).
'protected by' means one lot of letters goes inside another.
'poop' inserted within 'parter' is 'partpooper'.
'partpooper' enclosing 'y' is 'PARTY-POOPER'.
Can you help me to learn more?
(Another definition for party-pooper that I've seen is " One spoiling others' fun".)