Wearisome (6)
I believe the answer is:
boring
'wearisome' is the definition.
(synonyms)
This is all the clue.
(Other definitions for boring that I've seen before include "like watching paint dry?" , "Mentally wearying" , "Tedious, tiresome" , "Drilling can be tedious" , "Dull and repetitive, a turn-off" .)