Eyesore includes M1 block (6)
I believe the answer is:
stymie
'block' is the definition.
(synonyms)
'eyesore includes m1' is the wordplay.
'eyesore' becomes 'stye' (I've seen this before).
'includes' is an insertion indicator.
'm1' becomes 'mi' (resembles M1).
'stye' going around 'mi' is 'STYMIE'.
(Other definitions for stymie that I've seen before include "Block or thwart, as on golf course" , "Block the path of, as on golf green" , "Hinder the progress of (something)" , "Obstruction (originally at golf)" , "'Hinder, prevent progress (6)'" .)