Donald hides them, the devil! (5)
I believe the answer is:
demon
'the devil' is the definition.
(synonyms)
'donald hides them' is the wordplay.
'donald' becomes 'don' (I can't explain this - if you can you should believe this answer much more).
'hides' is an insertion indicator.
'them' becomes 'em' (abbreviation).
'don' going around 'em' is 'DEMON'.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Ghoul" , "beast" , "Bad spirit" , "Skilful person" , "Devil - master performer" .)