Is the slight lowering of the garment below the ankles when bending considered isbal, and must it be shortened further to avoid that, and what is the definition of isbal?

If your garments do not extend beyond the ankles when you stand upright, then there is no harm in them lowering when you bend, and this is not considered isbal (letting garments hang below the ankles). This is based on the Prophet's (peace be upon him) saying: "There is no harm or no sin in what is between it and the ankles." The prohibited isbal is that which is below the ankles.

If your garment does not extend beyond the ankles, you are not obligated to shorten it, unless you wish to shorten it to mid-calf, in which case the matter is broad, based on the Prophet's (peace be upon him) saying: "The believer's izra (lower garment) is to mid-calf, and there is no harm or no sin in what is between it and the ankles."