The most common and most likely answer to this crossword clue is the 5 letter word TAGUS
I've seen this crossword clue in The Guardian
T
A
G
U
S
More clues leading to the result TAGUS
- Turn gun on American banker in Portugal
- Follow American river
- Put a name to American river, longest in country
- Children’s game gets you and me in river
- European flower market’s ultimate sugar confection? No resistance!
- Children's game next to American river
- Name American river
- Saying to ourselves: ‘This is a very long river’
- Flower you and I name first
Buddy explains!
The straight part
We know that "river, longest in country" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case river, longest in country = tagus.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.