Mousy girl college rejected (5)
I believe the answer is:
timid
'mousy' is the definition.
('timid' can be similar in meaning to 'mousy')
'girl college rejected' is the wordplay.
'girl' becomes 'di' (**).
'college' becomes 'mit' (I've seen this in other clues).
'rejected' says the letters should be written backwards.
'di'+'mit'='dimit'
'dimit' reversed gives 'TIMID'.
(Other definitions for timid that I've seen before include "Diffident, faint-hearted" , "Pusillanimous" , "Mouse" , "Shrinking" , "Easily alarmed, shy" .)