Most uninteresting (7)
I believe the answer is:
dullest
'most uninteresting' is the definition.
('dull' can be similar in meaning to 'uninteresting')
This is all the clue.
(Other definitions for dullest that I've seen before include "Least interesting" , "that's not at all interesting" , "Least shiny" , "Most unintelligent" , "Most insensitive" .)