Become boring (4)
I believe the answer is:
pall
'become boring' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pall that I've seen before include "shadow" , "Cloth spread over a coffin" , "Become less interesting" , "Grow stale; coffin cloth" , "Cloth to cover a coffin" .)