Bother few who are upsetting (11)
I believe the answer is:
troublesome
'are upsetting' is the definition.
(thesaurus)
'bother few' is the wordplay.
'bother' becomes 'trouble' ('trouble' can be a synonym of 'bother').
'few' becomes 'some' ('some' is associated in meaning with 'few'**).
'trouble'+'some'='TROUBLESOME'
'who' acts as a link.
(Other definitions for troublesome that I've seen before include "Hard" , "Trying" , "likely to cause ructions" , "problematic" , "Causing annoyance" .)