Peers squeal when upset (6)
I believe the answer is:
equals
'peers' is the definition.
('equal' can be a synonym of 'peer')
'squeal when upset' is the wordplay.
'when upset' is an anagram indicator.
'squeal' with letters rearranged gives 'EQUALS'.
(Other definitions for equals that I've seen before include "Persons of the same rank, for instance" , "Peer group" , "Is identical in quantity, value" , "Those with the same status, rights, etc." , "They're at the same level" .)