Ugly spectacle one witnessed reportedly (7)
I believe the answer is:
eyesore
'ugly spectacle' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'one witnessed reportedly' is the wordplay.
'one' becomes 'eye' (I can't justify this - if you can you should give a lot more credence to this answer).
'witnessed' becomes 'saw' ('see' can be a synonym of 'witness').
'reportedly' indicates a 'sounds like' (homophone) clue.
'saw' is a homophone of 'sore'.
'eye'+'sore'='EYESORE'
(Other definitions for eyesore that I've seen before include "Ugly and offensive sight" , "Something ugly and offensive" , "Something that's ugly" , "Ugly item" , "Blot on the landscape" .)