Not letting anyone see one's heavy defeat (6)
I believe the answer is:
hiding
This clue is a double definition.
'not letting anyone see one's' is the first definition.
(hiding oneself from view)
'heavy defeat' is the second definition.
(hiding can mean a dramatic defeat)
(Other definitions for hiding that I've seen before include "Concealing oneself - good thrashing" , "Place of concealment" , "Harm, damage" , "Physical beating" , "Beating - retreat" .)