Henry leaving posh school gives pointer (5)
I believe the answer is:
arrow
'pointer' is the definition.
(I know that pointer can be written as arrow)
'henry leaving posh school' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'leaving' indicates named letters should be taken away.
'posh school' becomes 'harrow' (UK public school).
'harrow' with 'h' taken away is 'ARROW'.
'gives' acts as a link.
(Other definitions for arrow that I've seen before include "what may have killed Harold?" , "Weapon from bow" , "Pointer; weapon" , "Move swiftly" , "Sharp-tipped missile" .)