Woman has university student to invalidate (5)
I believe the answer is:
annul
'to invalidate' is the definition.
('annul' can be a synonym of 'invalidate')
'woman has university student' is the wordplay.
'woman' becomes 'ann' (woman's name).
'has' means one lot of letters go next to another.
'university' becomes 'U' (abbreviation for university).
'student' becomes 'L' (as in L-plates for learner drivers).
'ann'+'u'+'l'='ANNUL'
(Other definitions for annul that I've seen before include "Declare void" , "Make invalid" , "Countermand" , "Cancel, abolish" , "Recall" .)