Chief at heart persistently stunned (6)
I believe the answer is:
aghast
'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" .)