Rascals swindle prosperous outsiders (6)
I believe the answer is:
scamps
'rascals' is the definition.
('scamp' can be a synonym of 'rascal')
'swindle prosperous outsiders' is the wordplay.
'swindle' becomes 'scam' (scam is a kind of swindle).
'outsiders' means to remove the middle letters (only the letters on the outside of the word remain).
'prosperous' with its centre taken out is 'ps'.
'scam'+'ps'='SCAMPS'
(Other definitions for scamps that I've seen before include "Scallywags" , "Lively children" , "Naughty little rascals" , "'Rascals, imps (6)'" , "Mischevious young children" .)