Duke is protected by fort (4)
I believe the answer is:
fist
'duke' is the definition.
(put up your dukes)
'is protected by fort' is the wordplay.
'protected by' indicates putting letters inside.
'fort' becomes 'ft'.
'is' put into 'ft' is 'FIST'.
(Other definitions for fist that I've seen before include "Bunch of fives" , "Hit with a clenched one" , "One may deck in a ring" , "Punch dispenser" , "Tightly-closed hand" .)