Letter from Greece seen repeatedly in Philadelphia (3)
I believe the answer is:
phi
'letter from greece' is the definition.
(letter from the Greek alphabet)
'repeatedly in philadelphia' is the wordplay.
'repeatedly in' can mean two replacements for 'in'.
'in' indicates the answer is hidden within the clue.
'PHI' can be found hidden inside 'in philadelphia'.
'seen' acts as a link.
(Other definitions for phi that I've seen before include "Greek character" , "classic character" , "One Greek" , "letter to Timon?" , "Letter from the Balkans" .)