Open a French letter (5)

I believe the answer is:

uncap

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'open' is the definition.
(I've seen this in another clue)

'a french letter' is the wordplay.
'a french' becomes 'un' ('a' in French).
'letter' becomes 'cap' (I can't justify this - if you can you should give a lot more credence to this answer).
'un'+'cap'='UNCAP'

Can you help me to learn more?

(Other definitions for uncap that I've seen before include "Take the top of the bottle" , "Take the lid off" , "Remove a top" , "Remove the lid" , "Remove a cover from" .)

I've seen this clue in The Independent.

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