Completely ousting leader of college (3)
I believe the answer is:
all
'completely' is the definition.
(synonyms)
'ousting leader of college' is the wordplay.
'ousting leader of' means to remove the first letter.
'college' becomes 'hall'.
'hall' with its first letter removed is 'ALL'.
(Other definitions for all that I've seen before include "and 16 Across and 23 Across: Even so" , "The lot, everyone" , "that's as many as there are" , "Passage in a formal garden" , "Completely, everything" .)