Greek character appearing in amphitheatre (3)
I believe the answer is:
phi
'greek character' is the definition.
(letter from the Greek alphabet)
'in amphitheatre' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PHI' is hidden within 'amphitheatre'.
'appearing' acts as a link.
(Other definitions for phi that I've seen before include "letter to Timon?" , "Twenty First Greek letter, pronounced as f" , "exotic character" , "Greek letter in previous answer" , "Greek letter; sounds like expression of disgust" .)