Set off from posh school in time (8)
I believe the answer is:
detonate
'set off' is the definition.
(synonyms)
'posh school in time' is the wordplay.
'posh school' becomes 'Eton'.
'in' is an insertion indicator.
'time' becomes 'date' (date is a kind of time).
'eton' inserted within 'date' is 'DETONATE'.
'from' is the link.
(Other definitions for detonate that I've seen before include "Blow up, explode" , "Start 15 [EXPLOSION]" , "Set off explosive device" , "Trigger" , "let off" .)