Further extra to the other side (8)
I believe the answer is:
moreover
'further' is the definition.
(I've seen this before)
'extra to the other side' is the wordplay.
'extra' becomes 'more' (both can mean additional).
'to the other side' becomes 'over' (over on the other side).
'more'+'over'='MOREOVER'
(Other definitions for moreover that I've seen before include "Besides, in addition" , "as well" , "Furthermore, in addition" .)