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