Clear area in emergency, initially — very critical (8)
I believe the answer is:
evacuate
'clear' is the definition.
(thesaurus)
'area in emergency initially very critical' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'in' indicates putting letters inside.
'initially' suggests taking the first letters.
'very' becomes 'v' (abbreviation).
'critical' becomes 'acute' (similar in meaning).
The first letter of 'emergency' is 'e'.
'e'+'v'+'acute'='evacute'
'a' inserted inside 'evacute' is 'EVACUATE'.
(Other definitions for evacuate that I've seen before include "Clear (dangerous building)" , "Leave unpopulated" , "Remove from a place of danger" , "Withdraw from (a place)" , "Make people leave a dangerous location" .)