University Department involved in colourful college festivity (5)
I believe the answer is:
gaudy
'colourful college festivity' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'university department involved in' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'department' becomes 'd' (abbreviation eg in MOD Ministry of Defence).
'involved' becomes 'gay' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'u'+'d'='ud'
'ud' inserted inside 'gay' is 'GAUDY'.
Can you help me to learn more?
(Other definitions for gaudy that I've seen before include "college party" , "Flashy" , "Garish" , "celebrations in college" , "Extravagantly showy; college function" .)