Rascal first in prison after swindle (5)
I believe the answer is:
scamp
'rascal' is the definition.
(I know that rascal can be written as scamp)
'first in prison after swindle' is the wordplay.
'first in prison' becomes 'p' (1st letter of 'prison').
'after' is a charade indicator (letters next to each other).
'swindle' becomes 'scam' (scam is a kind of swindle).
'p' put after 'scam' is 'SCAMP'.
(Other definitions for scamp that I've seen before include "pickle" , "Tearaway" , "Troublemaker" , "Mischievous child (colloq.)" , "villain" .)