Main safety equipment activated, initially (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'safety equipment activated initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'safety equipment activated' is 'SEA'.
(Other definitions for sea that I've seen before include "The Aegean, for example" , "Caspian, e.g." , "All at . . . - totally confused" , "eg Galilee" , "Body of saltwater" .)