Extra code missing ... (4)

I believe the answer is:

'extra' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for more that I've seen before include "Greater quantity or number" , "A greater amount" , "See 9" , "Extra (for Sir Thomas?)" , "What Oliver wanted" .)

I've seen this clue in The Guardian.

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