Uncle again gets the wrong sum (5)
I believe the answer is:
remus
'uncle' is the definition.
(I've seen this before)
'again gets the wrong sum' is the wordplay.
'again' becomes 're'.
'gets' is a charade indicator (letters next to each other).
'the wrong' indicates an anagram.
'sum' is an anagram of 'mus'.
're'+'mus'='REMUS'
(Other definitions for remus that I've seen before include "twin brother" , "One of the founders of Rome" , "Co-founder of Rome" , "Uncle --, US narrator" , "Twin founder of Rome" .)