Set off from posh school, getting into Escort (8)
I believe the answer is:
detonate
'set off' is the definition.
(synonyms)
'posh school getting into escort' is the wordplay.
'posh school' becomes 'Eton'.
'getting into' indicates putting letters inside.
'escort' becomes 'date' (escort can mean a date or partner).
'eton' inserted inside 'date' is 'DETONATE'.
'from' acts as a link.
(Other definitions for detonate that I've seen before include "Trigger" , "Start 15 [EXPLOSION]" , "Cause to explode" , "Set off explosive device" , "Discharge" .)