Principal knight is harsh (5)
I believe the answer is:
stark
'harsh' is the definition.
(thesaurus)
'principal knight' is the wordplay.
'principal' becomes 'star' ('star' can be a synonym of 'principal').
'knight' becomes 'k' (**).
'star'+'k'='STARK'
'is' is the link.
(Other definitions for stark that I've seen before include "Bleak" , "Blunt; grim" , "Absolutely crazy" , "Severely simple" , "Severe in appearance" .)