Vessels filled with last of Scotch — cheers! (6)
I believe the answer is:
thanks
'cheers' is the definition.
('cheers' can informally mean 'thank you')
'vessels filled with last of scotch' is the wordplay.
'vessels' becomes 'tanks' (tank is a kind of vessel).
'filled with' indicates putting letters inside.
'last of' indicates one should take the final letters.
The final letter of 'scotch' is 'h'.
'tanks' going around 'h' is 'THANKS'.
(Other definitions for thanks that I've seen before include "Much obliged!" , "Brief expression of appreciation" , "Ta" , "I appreciate that" , "Expressions of gratitude" .)