The high quality of many a mug (5)
I believe the answer is:
class
'the high quality' is the definition.
(I've seen this before)
'many a mug' is the wordplay.
'many a' becomes 'cl' (I can't justify this - if you can you should believe this answer much more).
'mug' becomes 'ass' (I've seen this before).
'cl'+'ass'='CLASS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for class that I've seen before include "Order - quality" , "Type; social level" , "Group of students" , "Form; stylishness" , "Elegance - form" .)