Ruin a good nurse over interns (4)
I believe the answer is:
undo
'ruin' is the definition.
(someone's undoing is their ruin)
'a good nurse over interns' is the wordplay.
I cannot quite see how this works, but
'over' could be 'o' (cricket abbreviation) and 'o' is located in the answer.
The remaining letters 'und' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for undo that I've seen before include "Disentangle" , "Cancel, annul; open" , "set free" , "Open button or ruin" , "Ruin or loosen ties" .)