Fraction of this clue plus this letter? (6)
I believe the answer is:
eighth
'fraction' is the definition.
(I know that eighth is a type of simple fraction)
'this clue plus this letter?' is the wordplay.
'this clue plus' becomes 'eight' (I can't justify this - if you can you should give a lot more credence to this answer).
'this letter?' becomes 'h'.
'eight'+'h'='EIGHTH'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for eighth that I've seen before include "Twelve and a half percent" , "Arm muscle" , "Half a quarter" , "12.5 per cent" , "One after seventh" .)