So Cretan possibly is forefather (8)
I believe the answer is:
'is forefather' is the definition.
(I've seen this before)
'so cretan possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'socretan' with letters rearranged gives 'ANCESTOR'.
(Other definitions for ancestor that I've seen before include "one is up a tree" , "Person from whom one is descended" , "One of one's forebears" , "Forefather" , "Adam was our first one" .)