Lively plonk quaffed by Sunday on vacation (6)
I believe the answer is:
sparky
'lively' is the definition.
(I've seen this before)
'plonk quaffed by sunday on vacation' is the wordplay.
'plonk' becomes 'park'.
'quaffed by' is an insertion indicator (inserted letters are drunk or quaffed by others).
'on vacation' says to hollow out the word (remove centre letters) (after the inside letters have vacated the word).
'sunday' with its middle removed is 'sy'.
'park' put inside 'sy' is 'SPARKY'.
(Other definitions for sparky that I've seen before include "jaunty" , "Lively, high-spirited" , "Clever and enjoyable to be with" , "Spirited" .)