Moody nag keeps cards close to chest? (4,5)
I believe the answer is:
dark horse
'cards close to chest?' is the definition.
I can't judge whether this definition defines the answer.
'moody nag' is the wordplay.
'moody' becomes 'dark' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nag' becomes 'horse' (a nag is an old horse).
'dark'+'horse'='DARK HORSE'
'keeps' acts as a link.
Can you help me to learn more?
(Other definitions for dark horse that I've seen before include "Black Beauty, for instance" , "Unfancied winner" , "unexpectedly successful entry" , "Unexpected (possible) winner" , "Candidate, not well known" .)