The value of a letter or two (5)
I believe the answer is:
worth
'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" .)