Yes the MEL only applies on the ground (or helideck), if something happens in flight you have an emergency checklist written for that very purpose (hopefully). If you therefore land (anywhere) with the fault, you then have to refer to the MEL to see if you can take off again. Depending on the fault, the MEL may say the item "may be inoperative for VFR" or, as in this case, "may be inoperative provided only one flight has been made since the fault was discovered". This means (normally after discussion with your ops and engineering people) you can take off again; the flight during which the fault occurred does not count as the "one flight since the fault was discovered". Of course you may decide, or your checklist may tell you, to "land as soon as practical" when you discover the fault, in which case you would turn round and fly back to base, at which point it becomes Engineering's problem. Does that all make sense?