Drone knocked back double shot of liquor (6)
I believe the answer is:
murmur
'drone' is the definition.
(thesaurus)
'knocked back double shot of liquor' is the wordplay.
'knocked back' says the letters should be written in reverse.
'double shot of' becomes 'rum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'liquor' becomes 'rum' (rum is a kind of liquor).
'rum'+'rum'='rumrum'
'rumrum' in reverse letter order is 'MURMUR'.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Low continuous sound" , "Quietly spoken utterance; heart defect" , "Mumbled expression of malcontent" , "Undertone" , "Low, indistinct speech" .)