Grotesque mouthwash to drink only on vacation (8)
I believe the answer is:
gargoyle
'grotesque' is the definition.
(I've seen this in another clue)
'mouthwash to drink only on vacation' is the wordplay.
'mouthwash' becomes 'gargle' (synonyms).
'to drink' is an insertion indicator.
'on vacation' says to hollow out the word (remove centre letters) (after the inside letters have vacated the word).
'only' with its middle taken out is 'oy'.
'gargle' placed around 'oy' is 'GARGOYLE'.
(Other definitions for gargoyle that I've seen before include "Medieval gutter spout made of sculpted grotesque figure" , "Ugly cathedral feature" , "Grotesque projection" , "Spout ending in a grotesquely carved figure" , "Ugly person's" .)