Brazilian port’s endless civil unrest (3)
I believe the answer is:
rio
'brazilian port's' is the definition.
(Rio de Janeiro)
'endless civil unrest' is the wordplay.
'endless' means to remove the last letter.
'civil unrest' becomes 'riot' (rioting is a type of civil unrest).
'riot' with its final letter removed is 'RIO'.
(Other definitions for rio that I've seen before include "(Golfer) in the sand" , "Location of next Olympic Games, in short" , "Port, -- de Janeiro" , "The ... Grande borders Texas and Mexico" , "- Ferdinand, footballer" .)