Talk foolishly about old heretic being divine (13)
I believe the answer is:
prognosticate
'being divine' is the definition.
(I've seen this before)
'talk foolishly about old heretic' is the wordplay.
'talk foolishly' becomes 'prate' (I've seen this before).
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'heretic' becomes 'gnostic'.
'o'+'gnostic'='ognostic'
'prate' placed around 'ognostic' is 'PROGNOSTICATE'.
(Other definitions for prognosticate that I've seen before include "Predict" , "Foretell" , "divine" , "Arbitrary" , "Forecast" .)