Besides, it’s further across (8)
I believe the answer is:
moreover
'besides' is the definition.
(synonyms)
'further across' is the wordplay.
'further' becomes 'more' (synonyms).
'across' becomes 'over' (synonyms).
'more'+'over'='MOREOVER'
'it's' is the link.
(Other definitions for moreover that I've seen before include "as well" , "Furthermore, in addition" , "Besides, in addition" .)