Set of letters upsetting Beth, a pal (8)
I believe the answer is:
alphabet
'set of letters' is the definition.
(I've seen this before)
'upsetting beth a pal' is the wordplay.
'upsetting' is an anagram indicator.
'beth'+'a'+'pal'='bethapal'
'bethapal' with letters rearranged gives 'ALPHABET'.
(Other definitions for alphabet that I've seen before include "Confusing assortment of initials" , "Fundamental language components" , "Whole set of letters" , "lots of characters" , "Ordered set of letters" .)