Agreement boy should go to HE institution (6)
I believe the answer is:
unison
'agreement' is the definition.
(unison is a kind of agreement)
'boy should go to he institution' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'should go to' means one lot of letters go next to another.
'he institution' becomes 'uni'.
'son' after 'uni' is 'UNISON'.
(Other definitions for unison that I've seen before include "Musical agreement" , "Agreement or concord, as with choir" , "Sounding or singing together at the same pitch" , "Concordance" , "In agreement - like sound of choir" .)