Swill round, swallowing only in extremis? That’s grotesque (8)
I believe the answer is:
gargoyle
'grotesque' is the definition.
'swill round swallowing only in extremis?' is the wordplay.
'swill round' becomes 'gargle'.
'swallowing' means one lot of letters goes inside another.
'in extremis?' means to remove the middle letters (only the letters on the extremes are used).
'only' with its centre taken out is 'oy'.
'gargle' enclosing 'oy' is 'GARGOYLE'.
'that's' acts as a link.
(Other definitions for gargoyle that I've seen before include "Ugly carved figure on a building" , "Grotesque projection" , "Grotesque carved figure as ancient water-spout" , "Ugly person's" , "grotesque sight!" .)