Some fun duenna finds excessive (5)
I believe the answer is:
Here is my best explanation:
'excessive' is the definition.
('undue' can be a synonym of 'excessive')
'some fun duenna finds' is the subsidiary indication.
'some' indicates the answer is hidden within the clue (Some of the parts of the word are used).
'undue' is hidden within 'fun duenna finds'.
(Other definitions for undue that I've seen before include "Needless; excessive", "Excessive, unreasonable", "Lacking justification", "Excessively, unreasonably", "Inordinate", "Inappropriate - excessive", "Excessive amount", "improper", "too much" and "Improper".)