The Bear River in America? (4)
I believe the answer is:
'the bear' is the definition.
(Latin name for bear as in Ursa Major)
'river in america?' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'in' means one lot of letters goes inside another.
'america?' becomes 'U.S.A.' ('USA' can be a synonym of 'america').
'r' going within 'usa' is 'URSA'.
(Other definitions for ursa that I've seen before include "Bear (constellation)" , "and 18 Down: The Great Bear" , "Major/Minor constellation" , "(and 11 across) Pole Star constellation" , "(& 19 across) Plough constellation" .)