Does blood clot reduce blood flow?
A blood clot can nearly completely or completely block an artery, in which case the blood flow will be reduced or stopped.

How do we know this?

Pulmonary Embolism (Merck Manuals):


  Pulmonary infarction is when some of the lung tissue does not receive
  enough blood flow and oxygen and appears on imaging studies to die due
  to blockage of a lung blood vessel by a pulmonary embolus.


An embolus is a blood clot that usually develops in the leg veins in individuals with deep venous thrombosis, detaches and travels to a certain pulmonary artery and blocks it.

Similarly, atheroma (atherosclerotic plaque) that builds up within the artery and only partially blocks it, can reduce the blood flow to the target organ. For example, a partial blockage of a coronary artery can result in decreased blood supply to the heart and consequently in angina pectoris or myocardial infarction. It is then the investigation called coronary angiography that can show that a certain coronary artery is partially blocked.

When a clot interferes with blood flow (Harvard.edu):


  In venous thromboembolism, a blood clot slows or stops the flow of
  blood through the veins...