A letter or two in code (5)

I believe the answer is:

morse

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'code' is the definition.
(Morse code)

'a letter or two' is the wordplay.
'a letter' becomes 'm'.
'two' becomes 'se' (I can't justify this - if you can you should give a lot more credence to this answer).
'm'+'or'+'se'='MORSE'

'in' is the link.

Can you help me to learn more?

(Other definitions for morse that I've seen before include "used to transmit messages" , "Radio code inventor" , "Code inventor, Samuel" , "Endeavour" , "10 [SLEUTH]" .)

I've seen this clue in the Evening Standard.

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