Greek character Penny is coming back (3)
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" .)