The nasty fellow says “Ah - you are out!” (5)
I believe the answer is:
yahoo
'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" .)