Some fun duenna finds excessive (5)
I believe the answer is:
undue
'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 "More than appropriate" , "Extreme" , "Improper" , "Needless; excessive" , "Not to be paid" .)