Some questions mother brought up to undermine former spouse (4)
I believe the answer is:
exam
'some questions' is the definition.
(a set of questions)
'mother brought up to undermine former spouse' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'brought up' shows that the letters should be reversed in order (in a down clue, letters go up).
'to undermine' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'former spouse' becomes 'ex' (an ex-spouse).
'ma' written backwards gives 'am'.
'am' after 'ex' is 'EXAM'.
(Other definitions for exam that I've seen before include "'Test, in short (4)'" , "Important test" , "One test" , "paper" , "(Academic) test" .)