Assembly dismissing University document (4)
'document' is the definition.
(form is a kind of document)
'assembly dismissing university' is the wordplay.
'assembly' becomes 'forum' (synonyms).
'dismissing' indicates named letters should be taken away.
'university' becomes 'u' (abbreviation for university).
'forum' with 'u' taken away is 'FORM'.
(Other definitions for form that I've seen before include "Make", "accommodation for hare", "Configuration; type", "etiquette", "Fitness".)