Unsuccessful (6)
I believe the answer is:
futile
'unsuccessful' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for futile that I've seen before include "Trifling and ineffectual" , "Ineffectual,in vain" , "Useless, a waste of effort" , "In vain, a waste of time" , "Ineffectual, a waste of time" .)