Dismayed, a spook has a change of heart (6)
I believe the answer is:
aghast
'dismayed' is the definition.
(I know that dismayed can be written as aghast)
'a spook has a change of heart' is the wordplay.
'a spook has' becomes 'agha' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a change' becomes 'bustle' (bustle is a kind of change).
'of heart' indicates the central letters (I've seen 'heart' mean this).
The central letters of 'bustle' are 'st'.
'agha'+'st'='AGHAST'
Can you help me to learn more?
(Other definitions for aghast that I've seen before include "Horror-struck" , "Filled with horror or shock" , "Filled with consternation or horror" , "Thunderstruck" , "Taken aback with horror" .)