Mostly going nowhere is dull (6)
I believe the answer is:
deaden
'is dull' is the definition.
(I've seen this in another clue)
'mostly going nowhere' is the wordplay.
'mostly' means to remove the last letter.
'going nowhere' becomes 'dead end' (I've seen this before).
'deadend' with its final letter removed is 'DEADEN'.
(Other definitions for deaden that I've seen before include "Kill" , "Make insensitive" , "Blunt, numb" , "Muffle impact" , "Benumb" .)