One backing a horse it may be added (5)

I believe the answer is:

rider

I believe this is a double definition.

'one backing a horse' is the first definition.
(I've seen this before)

'it may be added' is the second definition.
(I've seen this before)

(Other definitions for rider that I've seen before include "Supplementary clause" , "extra weight needed for balance?" , "Supplementary verdict brought in by jury" , "Condition added to something already agreed" , "Revere, for example" .)

I've seen this clue in The Globe and Mail.

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