Let Down is a somewhat antiquated term for :Instrument approach", meaning a procedure to descend when visual reference (due to cloud or fog) is not available.
In earlier times, these consisted of prodedures to ensure the aircraft was over safe terain, then "Letting Down" to a safe altitude, hopefully below the cloud.
Precision or non-precision approach is the more usually used terminology these days.
HOWEVER,
As you are obviously a keen student, I hope you won't mind me pointing out an easily made error in your statement. You do not lose (NOTE not "LOOSE") height because of "Decreased lift due to decreased speed as per lift formula."
You increase AofA and thus Cl so that, in a steady descent, Lift still equals Weight. If this were not so, the aircraft would continuosly ACCELRATE vertically downward, which it does not.
It descends because THRUST is no longer sufficient to balance DRAG in level flight. As the aircraft descends, the available Thrust increases, until it is sufficient for level flight.
Old Fella was trying to be funny. Probably doesn't translate well to either your generation or your native language.