Stop row involving Greek character (6)
I believe the answer is:
detain
'stop' is the definition.
(I've seen this before)
'row involving greek character' is the wordplay.
'row' becomes 'din' (both can mean a loud disturbance).
'involving' is an insertion indicator.
'greek character' becomes 'eta' (letter from the Greek alphabet).
'din' enclosing 'eta' is 'DETAIN'.
(Other definitions for detain that I've seen before include "Ate din (anag.)" , "Hold back - arrest" , "Hold back, perhaps for questioning" , "Hold in detention" , "Hang on to" .)