Fatiguing; boring (8)
I believe the answer is:
tiresome
I believe this clue is a double definition.
'fatiguing' is the first definition.
(I've seen this in another clue)
'boring' is the second definition.
(synonyms)
(Other definitions for tiresome that I've seen before include "Rather boring" , "Causing impatience" , "irritating" , "So metier may be boring (8)" , "Causing boredom" .)