Become wearisome (4)
I believe the answer is:
'become wearisome' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pall that I've seen before include "Become less appealing" , "A dark cover; cease to interest" , "Become wearisome" , "tedious here" , "Cloth over a coffin - become insipid" .)