Small role with extreme action for Ancient Greek (7)
I believe the answer is:
spartan
'for ancient greek' is the definition.
(I've seen this in another clue)
'small role with extreme action' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'role' becomes 'part' (eg a part or role in a film).
'with' means one lot of letters go next to another.
'extreme' suggests removing the centre.
'action' with its middle taken out is 'an'.
's'+'part'+'an'='SPARTAN'
(Other definitions for spartan that I've seen before include "Indifferent to comfort" , "Ascetic - ancient Greek" , "Severe" , "Very strict - austere" , "Lacking luxury, austere" .)