Clue: Guardian nursing river country (5)
Answer: URSUS
Explanation: Guardian nursing river country (5)
R in US (Guardian) + U[nited] S[tates]. Either there is no definition here, or it's an &lit that I don't understand. There's a minor character called Ursus who is a bodyguard (="nurse"?) in the novel *Quo Vadis*, but that seems an unlikely explanation, to say the least