A river? Certainly not! But it is! (4)

I believe the answer is:

arno

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

'it is' is the definition.
The definition suggests a singular noun which matches the answer.

'a river? certainly not' is the wordplay.
'river?' becomes 'r' (abbreviation for river).
'certainly not' becomes 'no'.
'a'+'r'+'no'='ARNO'

'but' acts as a link.

Can you help me to learn more?

(Other definitions for arno that I've seen before include "River flowing through Florence and Pisa" , "25's river" , "Banker" , "River of 11 across" , "River in central Italy" .)

I've seen this clue in The Guardian.

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.