Scoundrel put in Tower as callous (6)
I believe the answer is:
rascal
'scoundrel' is the definition.
(I know that rascal is a type of scoundrel)
'in tower as callous' is the wordplay.
'in' indicates a hidden word.
'RASCAL' can be found hidden inside 'tower as callous'.
'put' is the link.
(Other definitions for rascal that I've seen before include "Rogue of a lascar" , "A scoundrel or scamp" , "Scoundrel or mischievous child" , "Cad" , "Reprobate" .)