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 wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'UNDUE' can be found hidden inside '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".)