Set off from public school during particular day (8)
I believe the answer is:
detonate
'set off' is the definition.
(synonyms)
'public school during particular day' is the wordplay.
'public school' becomes 'eton' (Eton College).
'during' means one lot of letters goes inside another.
'particular day' becomes 'date' (I've seen this before).
'eton' put within 'date' is 'DETONATE'.
'from' is the link.
(Other definitions for detonate that I've seen before include "Discharge" , "let off" , "Set off explosive device" , "Blow up, explode" , "Start 15 [EXPLOSION]" .)