Flog prisoner in cruel castle regularly (10)
I believe the answer is:
flagellate
'flog' is the definition.
(flagellating is a kind of flogging)
'prisoner in cruel castle regularly' is the wordplay.
'prisoner' becomes 'lag' (slang term).
'in' indicates putting letters inside.
'cruel' becomes 'fell' (Fell means capable off).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'castle' are 'ate'.
'lag' going into 'fell' is 'flagell'.
'flagell'+'ate'='FLAGELLATE'
(Other definitions for flagellate that I've seen before include "Scourge" , "Whip" .)