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