Prophetic leader agrees to alter forecast (7)
I believe the answer is:
presage
'forecast' is the definition.
(I know that forecast can be written as presage)
'prophetic leader agrees to alter' is the wordplay.
'leader' suggests taking the first letters.
'to alter' indicates an anagram.
The initial letter of 'prophetic' is 'p'.
'agrees' is an anagram of 'resage'.
'p'+'resage'='PRESAGE'
(Other definitions for presage that I've seen before include "Be a sign or warning of (an event)" , "Forecast, foreshadow" , "Be a sign of (something)" , "Foretell" , "Be an advance sign of, portend" .)