Penalty for a swindler is to be put in prison (7)
I believe the answer is:
confine
'put in prison' is the definition.
'confine' can be an answer for 'prison' (thesaurus). I am not certain of the 'put in' bit.
'penalty for a swindler' is the wordplay.
'penalty' becomes 'fine' (a financial penalty).
'for' means one lot of letters go next to another (I've seen this in other clues).
'a swindler' becomes 'con' (I've seen this in other clues).
'fine' put after 'con' is 'CONFINE'.
'is to be' acts as a link.
Can you help me to learn more?
(Other definitions for confine that I've seen before include "Keep within limits" , "Restrict (to an area)" , "Shut in" , "Limit" , "Imprison" .)