Initially Prince Harry agrees forecast (7)
I believe the answer is:
presage
'forecast' is the definition.
(I know that forecast can be written as presage)
'initially prince harry agrees' is the wordplay.
'initially' says to take the initial letters.
'harry' indicates anagramming the letters (harry can mean to attack or spoil).
The first letter of 'prince' is 'p'.
'agrees' anagrammed gives 'resage'.
'p'+'resage'='PRESAGE'
(Other definitions for presage that I've seen before include "Portent" , "Foretell" , "Be an advance sign of, portend" , "Be a sign of (something)" , "Be a sign or warning of (an event)" .)