Vivacity almost completely suppressed by an analgesic (7)
I believe the answer is:
aspirin
'analgesic' is the definition.
(I know that aspirin is a type of analgesic)
'vivacity almost completely suppressed by an' is the wordplay.
'vivacity' becomes 'spirit' (I've seen this in another clue).
'almost completely' means to remove the last letter.
'suppressed by' means one lot of letters goes inside another (I've seen this in other clues).
'spirit' with its final letter taken off is 'spiri'.
'spiri' going within 'an' is 'ASPIRIN'.
(Other definitions for aspirin that I've seen before include "Headache killer?" , "Commonly used pain-killer" , "Pain-relieving drug made in Paris" , "Headache pill" , "Anodyne" .)