In favour of enclosing University Square (4)
I believe the answer is:
four
'square' is the definition.
(four is a square number)
'in favour of enclosing university' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'enclosing' is an insertion indicator.
'university' becomes 'u' (abbreviation for university).
'for' going around 'u' is 'FOUR'.
(Other definitions for four that I've seen before include "Boundary hit (cricket)" , "An even number" , "teatime?" , "Set of bridge players?" , "Number of players in a doubles game" .)