Unjustifiably (6)
I believe the answer is:
unduly
'unjustifiably' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unduly that I've seen before include "Disproportionately or excessively" , "Immoderately" , "Inordinately" , "Unjustifiably" , "Excessive, unjustified" .)