Sordid letters to a girl (5)

I believe the answer is:

seamy

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'sordid' is the definition.
(synonyms)

'letters to a girl' is the wordplay.
'letters' becomes 'se' (I can't explain this - if you can you should believe this answer much more).
'to a girl' becomes 'amy'.
'se'+'amy'='SEAMY'

Can you help me to learn more?

(Other definitions for seamy that I've seen before include "Squalid" , "Disreputable (side)" , "Sleazy, sordid" , "Marked with joins; disreputable" , "Sordid, disreputable" .)

I've seen this clue in the Evening Standard.

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