Ineffectual, in vain (6)
I believe the answer is:
futile
I believe this is a double definition.
'ineffectual' is the first definition.
(I know that ineffectual can be written as futile)
'in vain' is the second definition.
(I've seen this before)
(Other definitions for futile that I've seen before include "Ineffective" , "Useless, producing no results" , "Ineffectual, unavailing" , "Fruitless" , "Useless or vain" .)