College boy in complete agreement (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(I've seen this before)
'college boy' is the wordplay.
'college' becomes 'uni' (I've seen this in other clues).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'uni'+'son'='UNISON'
'in' is the link.
(Other definitions for unison that I've seen before include "Close agreement" , "Harmony - public services' trade union" , "all players directed to the same pitch" , "Unanimity" , "In agreement - like sound of choir" .)