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