Crossword Genius

The value of a letter or two (5)

Ross

I believe the answer is:

worth

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'the value' is the definition.
(I've seen this before)

'a letter or two' is the wordplay.
'a letter' becomes 'w'.
'two' becomes 'th' (I can't explain this - if you can you should give a lot more credence to this answer).
'w'+'or'+'th'='WORTH'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for worth that I've seen before include "Throw (anag.)" , "Value (of something)" , "Merit - value" .)

I've seen this clue in the Evening Standard.
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