Couple regularly encountered in Douro (3)
I believe the answer is:
duo
'couple' is the definition.
(synonyms)
'regularly encountered in douro' is the wordplay.
'regularly encountered in' means one should take alternating letters (letters taken at regular intervals).
The alternate letters of 'douro' are 'DUO'.
(Other definitions for duo that I've seen before include "Two-person act" , "Double act" , "Pair of performers" , "Pair of musicians" , "Two players" .)