Open University refusal to admit daughter (4)
I believe the answer is:
undo
'open' is the definition.
(eg opening a bottle)
'university refusal to admit daughter' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'refusal' becomes 'no'.
'to admit' is an insertion indicator.
'daughter' becomes 'd' (genealogical abbreviation).
'no' enclosing 'd' is 'ndo'.
'u'+'ndo'='UNDO'
(Other definitions for undo that I've seen before include "Dismantle - destroy" , "Disentangle" , "Open button or ruin" , "Ruin or undermine" , "Take back" .)