Goes brown and awfully dark inside mugs (8)
I believe the answer is:
tankards
'mugs' is the definition.
(I've seen this in another clue)
'goes brown and awfully dark inside' is the wordplay.
'goes brown' becomes 'tas' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' becomes 'n' (common abbreviation for 'and').
'awfully' indicates anagramming the letters.
'inside' indicates putting letters inside.
'dark' with letters rearranged gives 'kard'.
'n'+'kard'='nkard'
'tas' placed around 'nkard' is 'TANKARDS'.
Can you help me to learn more?
(Other definitions for tankards that I've seen before include "One-handled beer mugs" , "Large one-handled drinking mugs" , "Large one-handled drinking vessels" , "Large beer mugs" .)