Dismiss a large part of design, or exclude (6)
I believe the answer is:
ignore
'dismiss' is the definition.
(synonyms)
'a large part of design or exclude' is the wordplay.
'a large part of' indicates the answer is hidden within the clue.
'IGNORE' is hidden within 'design or exclude'.
(Other definitions for ignore that I've seen before include "Brush off" , "Pass over" , "Fail to notice (6)" , "Neglect" , "Turn a blind eye to" .)