Refused to acknowledge (6)
I believe the answer is:
denied
'refused to acknowledge' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for denied that I've seen before include "Refused to admit and refused access to" , "Indeed (anag)" , "Refused to acknowledge, said it's not true" , "Contradicted" , "Refused to give (someone something)" .)