Crossword Genius

Chief at heart persistently stunned (6)

Ross

I believe the answer is:

aghast

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'stunned' is the definition.
(I've seen this before)

'chief at heart persistently' is the wordplay.
'chief at' becomes 'agha' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'heart' says to take the centre.
The central letters of 'persistently' are 'st'.
'agha'+'st'='AGHAST'

Can you help me to learn more?

(Other definitions for aghast that I've seen before include "What's carried in bag has teacher" , "Shocked, dumbfounded" , "Appalled, filled with consternation" , "Deeply dismayed" , "Taken aback with horror" .)

I've seen this clue in The Guardian.
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