Add a fifty to sum (5)
I believe the answer is:
total
'add' is the definition.
(I know that add can be written as total)
'a fifty to sum' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'sum' becomes 'tot' (synonyms).
'a'+'l'='al'
'al' put after 'tot' is 'TOTAL'.
(Other definitions for total that I've seen before include "Complete sum" , "Aggregate, sum" , "Aggregate amount" , "Complete; add up" , "Complete, entire amount" .)