"If on an IFR flight plan and the controller tells you in the middle of your flight to fly direct to your destination, does that mean to fly the flight plan you have been given or straight to your destination?" - Norman
“A clearance to proceed direct will always specify the point or fix to which you should fly. If ATC says proceed direct to your destination, then you should fly a direct course from your present position to your destination airport.
Clearance to proceed to a fix on your flight plan route and then to rejoin the route will have the phrase join victor xx, rest of route unchanged, or join victor xx, resume own navigation.
If you are on a RADAR vector, the controller must specify the point at which the vector ends and you are expected to resume your own navigation. For example – fly heading 360 intercept V1, resume own navigation.
If there is any doubt about your clearance, verify with the controller.”