Crossword Genius

The nasty fellow says “Ah - you are out!” (5)

Ross

I believe the answer is:

yahoo

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

'the nasty fellow' is the definition.
'yahoo' can be an answer for 'fellow' (I have seen 'crude fellow' mean 'yahoo' so perhaps 'fellow' could also mean 'yahoo'). I am not sure about the 'the nasty' bit.

'says ah you are out' is the wordplay.
I cannot really see how this works, but
'ah' is within the answer.
The remaining letters 'yoo' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for yahoo that I've seen before include "Swiftian ruffian" , "Coarse or brutish person" , "One of Dean Swift's brute people" , "Uncouth person" , "Race of brutes in Gulliver's Travels" .)

I've seen this clue in the Evening Standard.
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