Wine one is mistaken to be diffident about? (6)
I believe the answer is:
sherry
'wine' is the definition.
(sherry is a kind of wine)
'one is mistaken to be diffident about?' is the wordplay.
'one is mistaken' becomes 'err' (I can't justify this - if you can you should give a lot more credence to this answer).
'to be diffident' becomes 'shy' ('shy' can be a synonym of 'diffident').
'about?' indicates putting letters inside.
'err' placed into 'shy' is 'SHERRY'.
Can you help me to learn more?
(Other definitions for sherry that I've seen before include "Spanish drink" , "Spanish fortified wine" , "Alcoholic drink" , "Fortified wine originally from Spain" , "Wine from Spain's Jerez region, drunk as an aperitif" .)