Whelp on a river surface... (5)

I believe the answer is:

popup

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

'surface' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'whelp on a river' is the wordplay.
'whelp' becomes 'pup' (synonyms).
'on' says to put letters next to each other.
'a river' becomes 'Po' (the river Po in Italy).
'pup' put after 'po' is 'POPUP'.

Can you help me to learn more?

(Another definition for popup that I've seen is " What 11 [TOAST] might do".)

I've seen this clue in The Daily Express.

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