Start replacing tons with grams in correct code (5)

I believe the answer is:

debug

I'm a little stuck... Click here to teach me more about this clue!

'correct code' is the definition.
'debug' can be an answer for 'correct' (debugging is a kind of correcting). I am unsure of the 'code' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for debug that I've seen before include "Remove errors from a computer program" , "Fix (program) errors" , "Fix computer program problems" , "Revise programme" , "Remove (software) errors" .)

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