Crossword Genius

Greek character Penny is coming back (3)

Ross

I believe the answer is:

psi

'greek character' is the definition.
(letter from the Greek alphabet)

'penny is coming back' is the wordplay.
'penny' becomes 'p'.
'coming back' is a reversal indicator.
'is' in reverse letter order is 'si'.
'p'+'si'='PSI'

(Other definitions for psi that I've seen before include "Twenty-third letter of Greek alphabet" , "Pounds per square inch (abbr.); Greek letter" , "letter from abroad" , "Greek character" , "Greek letter; pressure unit" .)

I've seen this clue in The Telegraph.
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