A mystery man keeps her love (4)
I believe the answer is:
mona
'love' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
'a mystery man keeps her' is the wordplay.
'a mystery' indicates an anagram.
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'her' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'man' anagrammed gives 'mna'.
'mna' going around 'o' is 'MONA'.
Can you help me to learn more?
(Other definitions for mona that I've seen before include "Lisa in the Louvre" , "Female" .)