Like river or sea, nominally (3)

I believe the answer is:

red

I'm a little stuck... Click here to teach me more about this clue!

'like river' is the definition.
'Red' can be an answer for 'river' (Red River is an example). I am unsure of the 'like' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for red that I've seen before include "lefty" , "A colourful communist" , "Colour of blood" , "Crimson" , "River, its source in the US, flowing north to empty into L Winnipeg" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.