Forward a letter or two (5)
I believe the answer is:
forth
'forward' is the definition.
(I know that forward can be written as forth)
'a letter or two' is the wordplay.
'a letter' becomes 'f'.
'two' becomes 'th' (I can't justify this - if you can you should give a lot more credence to this answer).
'f'+'or'+'th'='FORTH'
Can you help me to learn more?
(Other definitions for forth that I've seen before include "Onward direction" , "running into view" , "River and estuary in eastern Scotland" , "Out and away -- UK shipping forecast area" , "Edinburgh's river" .)