On the quiet, I'm a devil! (3)
I believe the answer is:
imp
'a devil' is the definition.
(devilish creature)
'on the quiet i'm' is the wordplay.
'on' says to put letters next to each other.
'the quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'i'm' becomes 'im' ().
'p' after 'im' is 'IMP'.
(Other definitions for imp that I've seen before include "Mischievous child" , "A small mischievous devil" , "Minx" , "one likely to cause trouble" , "Small demon" .)