DVT
There is a risk of DVT in air travel. It is probably identical to the risk of car, bus and train travel, or the risk of sitting in a chair at home for the same period of time. It can be difficult to quantify as the symptoms are sometimes delayed.
The risk is one of immobilisation, and is not specific to the aviation environment. There are a number of medical conditions which also increase the risk of DVT.
Compression stockings may reduce the risk, particularly in susceptible individuals, and have little in the way of side effects. Drink plenty of water, not because the level of hydration makes much difference, but because it forces one to get up and walk around periodically.