Upheaval as Merlot replaced Medoc at first (9)
I believe the answer is:
maelstrom
'upheaval' is the definition.
(I have seen 'violent upheaval' mean 'maelstrom' so perhaps 'upheaval' could also mean 'maelstrom')
'as merlot replaced medoc at first' is the wordplay.
'replaced' is an anagram indicator.
'at first' indicates taking the first letters.
The initial letter of 'medoc' is 'm'.
'as'+'merlot'='asmerlot'
'asmerlot' with letters rearranged gives 'maelstro'.
'maelstro'+'m'='MAELSTROM'
(Other definitions for maelstrom that I've seen before include "Strong circular current of water" , "Norwegian whirlpool" , "Powerful circular current of water" , "extreme turbulence" , "Eddy" .)