This is an interesting situation when reading the new FIFA and UEFA guidelines and trying to interpret the meaning and importance of these 3 points:
The ability of an opponent to play the ball
It is quite easy to see that the attacker is impeding the movements of an opponent here, so does this mean that offside should certainly be called?
When we read the guidelines once more then we see that it says: "a clear action within playing distance which impacts the ability of the opponent to play the ball".
In reality the defender does not have a chance to play the ball here so this action does not actually happen in "playing distance" - therefore this could be interpreted as no offside. But as long as there is no direct answer to this clip then the assistant should be supported with offside (as it is similar to UEFA videos nr 1&2). It is a clip that would need to be explained by FIFA and UEFA.
For the assistant at the game in live this would rather not feel like an offside infringement and the assistant normally allows play to continue (as recommended in these cases).