Complete agreement with child attending university? (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(unison can mean total agreement or harmony)
'child attending university?' is the wordplay.
'child' becomes 'son' (male child).
'attending' means one lot of letters go next to another (to attend can mean to follow).
'university?' becomes 'uni' (informal abbreviation).
'son' put after 'uni' is 'UNISON'.
'with' is the link.
(Other definitions for unison that I've seen before include "A coincidence of musical notes" , "Sameness in pitch" , "There's agreement" , "with regard to the pitch" , "concert" .)