Treacherous Greek character retreats, shot outside (5)
I believe the answer is:
punic
'treacherous' is the definition.
(I know that treacherous can be written as punic)
'greek character retreats shot outside' is the wordplay.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'retreats' is a reversal indicator (I've seen 'retreating' mean this).
'shot' becomes 'pic' (short for picture).
'outside' means one lot of letters goes inside another.
'nu' in reverse letter order is 'un'.
'un' placed within 'pic' is 'PUNIC'.
(Other definitions for punic that I've seen before include "Relating to Carthage" , "Carthaginian" , "ancient language" .)