Total that is under 100 - someone helping with your golf? (6)

I believe the answer is:
caddie
'total that is under 100 someone helping with your golf?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'total that is under 100' is the wordplay.
'total' becomes 'add' (synonyms).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'100' becomes 'C' (Roman numeral).
'add'+'ie'='addie'
'addie' put after 'c' is 'CADDIE'.
Can you help me to learn more?
(Other definitions for caddie that I've seen before include "Golf course assistant" , "assist on course?" , "Padraig Harrington's brother-in-law Ronan Flood is also his . . . . . ." , "He's on course to help" , "club assistant" .)
