Board in grand public school (3,2)
I believe the answer is:
get on
'board' is the definition.
(synonyms)
'grand public school' is the wordplay.
'grand' becomes 'G' (G is short for ground as in a thousand).
'public school' becomes 'eton' (Eton College).
'g'+'eton'='GET ON'
'in' is the link.
(Other definitions for get on that I've seen before include "Board - prosper" , "Hit it off" , "Make progress; be friends" , "Board vehicle or make progress" , "Agree - embark - make progress" .)