Break the news briefly to Marsh knocking back wine glass (6)
I believe the answer is:
goblet
'wine glass' is the definition.
'goblet' can be an answer for 'glass' (goblet is a kind of glass). I'm not certain of the 'wine' bit.
'break the news briefly to marsh knocking back' is the wordplay.
'break the news briefly' becomes 'tel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to marsh' becomes 'bog' (I've seen this before).
'knocking back' shows that the letters should be reversed in order.
'tel'+'bog'='telbog'
'telbog' backwards is 'GOBLET'.
Can you help me to learn more?
(Other definitions for goblet that I've seen before include "small mouth?" , "Stemmed drinking glass" , "Large drinking glass with stem and base" , "Chalice" , "Drinking-glass with stem and foot" .)