See walls in posh school explode (8)
I believe the answer is:
detonate
'explode' is the definition.
(I know that explode can be written as detonate)
'see walls in posh school' is the wordplay.
'see' becomes 'date' (synonyms).
'walls in' means one lot of letters goes inside another (inserted letters are walled in).
'posh school' becomes 'Eton'.
'date' placed around 'eton' is 'DETONATE'.
(Other definitions for detonate that I've seen before include "Cause to explode" , "Trigger" , "Discharge" , "let off" , "Blow up, explode" .)