Bert is upset by lots of people (6)
I believe the answer is:
tribes
'lots of people' is the definition.
The answer and definition can be both groups as well as being plural nouns.
Perhaps you can see an association between them that I don't see?
'bert is upset' is the wordplay.
'upset' indicates anagramming the letters.
'bert'+'is'='bertis'
'bertis' with letters rearranged gives 'TRIBES'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for tribes that I've seen before include "Groups of families in traditional societies" , "races" , "Groups of people related by blood" , "Communities of primitive or nomadic groups" , "Racial groups living in communities headed by chiefs" .)