Most of the thing is dull (5)
I believe the answer is:
matte
'dull' is the definition.
(similar in meaning)
'most of the thing' is the wordplay.
'most of' means to remove the last letter.
'the thing' becomes 'matter' (synonyms).
'matter' with its final letter taken off is 'MATTE'.
'is' is the link.
(Other definitions for matte that I've seen before include "Mask used to add an image in front of camera" , "Mask used to block out areas of the image in filming" , "Unusually dull" , "Mask used in film; ET mat (anag.)" , "Mask used to record two images on same film" .)