“Absolutely simian!” and an exclamation of impatience (5)

I believe the answer is:

apish

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

'absolutely simian' is the definition.
Although both the answer and definition are adjectives, I don't see how one could define the other.

'an exclamation of impatience' is the wordplay.
'an' becomes 'a'.
'exclamation of impatience' becomes 'pish' (I can't justify this - if you can you should believe this answer much more).
'a'+'pish'='APISH'

'and' is the link.

Can you help me to learn more?

(Other definitions for apish that I've seen before include "Foolish; simian" , "Silly" , "Foolish; like a monkey" , "Stupid" , "Foppish" .)

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.