Democrat's gripped by sudden anxiety — it's all over (8)
I believe the answer is:
pandemic
'it's all over' is the definition.
'democrat's gripped by sudden anxiety' is the wordplay.
'democrat' becomes 'dem' (common abbreviation in US politics).
'gripped by' means one lot of letters goes inside another.
'sudden anxiety' becomes 'panic'.
'dem' going into 'panic' is 'PANDEMIC'.
(Other definitions for pandemic that I've seen before include "Existing everywhere" , "(Of a disease) affecting a wide area" , "Decamp in middle of world outbreak of disease" , "Occurring over a wide area" , "major disaster" .)