A juicy mug (3)
I believe the answer is:
sap
'mug' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sap that I've seen before include "Charlie" , "Weaken; tree fluid" , "tunnel" , "Juice; energy" , "Drain - vital fluid" .)