This swindle, on the quiet, is rascally (5)
I believe the answer is:
scamp
'this swindle on the quiet is rascally' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'this swindle on the quiet' is the wordplay.
'this swindle' becomes 'scam' (scam is a kind of swindle).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'scam'+'p'='SCAMP'
Can you help me to learn more?
(Other definitions for scamp that I've seen before include "little monkey" , "Scoundrel" , "Mischievous child (colloq.)" , "pickle" , "Little Devil" .)