Greek character nicked old Spanish coin (7)
I believe the answer is:
pistole
'old spanish coin' is the definition.
(historical type of coin)
'greek character nicked' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'nicked' becomes 'stole' (nick can mean to steal).
'pi'+'stole'='PISTOLE'
(Other definitions for pistole that I've seen before include "Old Spanish coin" , "Spanish gold coin" , "old gold coin" , "bit of gold" .)