By word of mouth or a letter (4)

I believe the answer is:

oral

'by word of mouth' is the definition.
(I've seen this before)

'or a letter' is the wordplay.
'letter' becomes 'l' (**).
'or'+'a'+'l'='ORAL'

(Other definitions for oral that I've seen before include "Relating to speech" , "Kind of examination" , "Of the voice" , "Unwritten examination" , "Ant; tourist (dial.)" .)

I've seen this clue in the Evening Standard.

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