Quarrel caused when posh school fails to open (5)
I believe the answer is:
arrow
'quarrel' is the definition.
(I've seen this before)
'posh school fails to open' is the wordplay.
'posh school' becomes 'harrow' (English private school).
'fails to open' means to remove the first letter.
'harrow' with its first letter taken off is 'ARROW'.
'caused when' acts as a link.
(Other definitions for arrow that I've seen before include "Weapon from bow" , "Bolt, quarrel" , "Pointed projectile" , "William Tell's missile" , "Sharp-tipped missile" .)