Do well at a good school (3,2)
I believe the answer is:
get on
'do well' is the definition.
(thesaurus)
'at a good school' is the wordplay.
'at' says to put letters next to each other.
'a good' becomes 'g' (abbreviation).
'school' becomes 'eton' (famous UK public school).
'g' put next to 'eton' is 'GET ON'.
(Other definitions for get on that I've seen before include "Board (a bus)" , "Agree - embark - make progress" , "Advance; prosper" , "Age; prosper" , "Proceed; prosper" .)