Uncle's about to sum up (5)
I believe the answer is:
remus
'uncle's' is the definition.
(I've seen this before)
'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'
(Other definitions for remus that I've seen before include "partner in City foundation" , "Early city planner" , "Brother of Romulus" , "One of the founders of Rome" , "murdered brother?" .)