Sister going round gallery found old Greek coin (6)
I believe the answer is:
stater
'old greek coin' is the definition.
(I've seen this before)
'sister going round gallery' is the wordplay.
'sister' becomes 'sr' (I've seen this in other clues).
'going round' is an insertion indicator.
'gallery' becomes 'tate' (I've seen this before).
'sr' placed around 'tate' is 'STATER'.
'found' acts as a link.
(Other definitions for stater that I've seen before include "One expressing clearly; ancient Greek coin" , "One specifying" , "I say" , "Greek currency as earlier" , "Old Greek coin" .)