A bit of a devil (4)
I believe the answer is:
evil
'a bit' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a devil' is the wordplay.
'EVIL' is hidden within the letters.
However, I am uncertain how this is indicated.
'of' is the link.
Can you help me to learn more?
(Other definitions for evil that I've seen before include "Wicked or immoral" , "Wicked - social problem" , "Extremely unpleasant" , "Badness, depravity" , "Demonic" .)