About to sum up the brother of Romulus (5)
I believe the answer is:
remus
'brother of romulus' is the definition.
(I've seen this in another clue)
'about to sum up' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'sum' backwards is 'mus'.
're'+'mus'='REMUS'
'the' acts as a link.
(Other definitions for remus that I've seen before include "Twin of Romulus" , "One responsible for capital" , "doomed wolf-suckling" , "Brother of 15" , "Suckled by a she-wolf, I was" .)