Dreadful dump entered by two bishops touring island (8)
I believe the answer is:
horrible
'dreadful' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for horrible that I've seen before include "'Ghastly, grisly (8)'" , "ghastly,grisly" , "'Ghastly, frightful (8)'" , "Unpleasant, dreadful" , "Appalling" .)