Full agreement boy must go to college (6)
I believe the answer is:
unison
'full agreement' is the definition.
(eg being in unison on a topic)
'boy must go to college' is the wordplay.
'boy' becomes 'son' (synonyms).
'must go to' means one lot of letters go next to another.
'college' becomes 'uni' (short for university).
'son' put after 'uni' is 'UNISON'.
(Other definitions for unison that I've seen before include "Public service trade union" , "Concordance" , "interval?" , "Unanimity" , "Concord, sounding or singing together" .)