What you can credit a mug with being (5)
I believe the answer is:
crass
'a mug with being' is the definition.
I can't tell whether this definition defines the answer.
'what you can credit' is the wordplay.
I cannot really understand how this works, but
'can' could be 'ass' (synonyms) and 'ass' is located in the answer.
'credit' could be 'cr' (abbreviation) and 'cr' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for crass that I've seen before include "Blundering" , "Indelicate" , "Insensitively stupid" , "Tactless, boorish" , "Asinine" .)