Calm down situation — a time to intervene (7)
I believe the answer is:
placate
'calm down' is the definition.
(I know that calm down is a is a more specific form of the action of placate)
'situation a time to intervene' is the wordplay.
'situation' becomes 'place' (place is a kind of situation).
'time' becomes 't' (abbreviation).
'to intervene' is an insertion indicator (intervene can mean to enter into).
'a'+'t'='at'
'place' going around 'at' is 'PLACATE'.
(Other definitions for placate that I've seen before include "Make calm, conciliate" , "Mollify, calm down" , "'Pacify, appease (7)'" , "Calm, pacify, appease" , "Mollify, appease" .)