Publication by students with classy, clean content, fantastic achievement (6,4)
I believe the answer is:
magnum opus
'fantastic achievement' is the definition.
'publication by students with classy clean' is the wordplay.
'publication' becomes 'mag' (abbreviation for magazine).
'by students' becomes 'nus' (I've seen this in other clues. I am not sure about the 'by' bit.**).
'with' indicates putting letters inside.
'classy' becomes 'u' (I've seen this in other clues).
'clean' becomes 'mop' (mopping is a kind of cleaning).
'u'+'mop'='umop'
'nus' enclosing 'umop' is 'numopus'.
'mag'+'numopus'='MAGNUM OPUS'
'content' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for magnum opus that I've seen before include "Great work" , "major achievement" , "Masterpiece" , "finest creation" , "one's best" .)