Seen in diagram, a random blot (3)
I believe the answer is:
mar
'blot' is the definition.
(thesaurus)
'seen in diagram a random' is the wordplay.
'seen in' says the answer is hidden in the clue.
'MAR' is hidden in the letters of 'diagram a random'.
(Other definitions for mar that I've seen before include "Spoil or deface" , "Blight" , "wreck" , "Ram into and spoil" , "'Impair, spoil (3)'" .)