Crooked scheme rascal detailed (4)
I believe the answer is:
scam
'crooked scheme' is the definition.
(a scam is a fraudulent scheme)
'rascal detailed' is the wordplay.
'rascal' becomes 'scamp' (synonyms).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'scamp' with its final letter taken off is 'SCAM'.
(Other definitions for scam that I've seen before include "Fraud, trick (slang)" , "A modern kind of swindle" , "An informal swindle" , "fiddle" , "Fraudulent scheme, swindle" .)