Quantity of chocolate surely transformed with a good piece of confectionery (6,5)
I believe the answer is:
barley sugar
'confectionery' is the definition.
(barley-sugar is a kind of confectionery)
'quantity of chocolate surely transformed with a good piece' is the wordplay.
'quantity of chocolate' becomes 'ba' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'transformed with' is an anagram indicator.
'good' becomes 'g' (abbreviation).
'piece' becomes 'r' (abbreviation for Rook).
'a'+'g'='ag'
'surely' mixed up with 'ag' gives 'rleysuga'.
'ba'+'rleysuga'+'r'='BARLEY-SUGAR'
'of' is the link.
Can you help me to learn more?
(Other definitions for barley sugar that I've seen before include "Twisted sweetmeat" , "Amber-coloured sweet" , "Confection (originally made from a cereal)" , "Real as Rugby (anag) - sweet" .)