First woman occasionally read letters for Post Office on top (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

1st and 3rd letters of ( occasionally ) “ read ” placed below ( … on top , in a down clue) [ P AND O ](or PO, letters/abbrev. for “Post Office”]. Defn: … who was created, according to Greek mythology.