Letter from Heraklion as acidic as can be? (3)
I believe the answer is:
phi
This clue is a double definition.
'letter from heraklion' is the first definition.
(I know that is part of Greek alphabet)
'as acidic as can be?' is the second definition.
(resembles pH 1 on the pH scale)
(Other definitions for phi that I've seen before include "Greek character" , "Greek letter, equivalent to F" , "Greek letter in previous answer" , "Letter from the Balkans" , "Greek letter sounding F" .)