Preclude (10)
'preclude' is the definition.
(I know that preclude can be written as rule out)
This is the entire clue.
(Other definitions for rule out that I've seen before include "Exclude from consideration" , "'Eliminate from consideration (4,3)'" , "discount" , "Exclude as a possibility" , "Eliminate, exclude as irrelevant" .)
(Other definitions for disqualify that I've seen before include "Exclude" , "Declare unsuitable" , "Remove from competition" .)