It makes one girl extremely eager to support college (7)
I believe the answer is:
unifier
'it makes one' is the definition.
(something which unifies people or makes them 'one')
'girl extremely eager to support college' is the wordplay.
'girl' becomes 'fi' (nickname for Fiona).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'college' becomes 'uni' (short for university).
'eager' with its centre taken out is 'er'.
'fi'+'er'='fier'
'fier' after 'uni' is 'UNIFIER'.
(Other definitions for unifier that I've seen before include "One who amalgamates, consolidator" , "One bringing together" , "It makes one" , "Binder" , "Type who joins in" .)