Rascal's detailed deception (4)
I believe the answer is:
scam
'deception' is the definition.
(I've seen this in another clue)
'rascal's detailed' is the wordplay.
'rascal' becomes 'scamp' (synonyms).
'detailed' means to remove the last letter (the tail of the word is removed).
'scamp' with its last letter removed is 'SCAM'.
(Other definitions for scam that I've seen before include "Fraudulent trick or swindle" , "Fraudulent action" , "ploy" , "Racket" , "A modern kind of swindle" .)