what is meaning of wavelength in very short conductor length
The wavelength of the signal doesn't depend on the length of the conductor it's transmitted over. You could say, it's the distance that would be between two maxima if the line was long enough to contain the full wavelength. If the conductor is short you may not be able to measure the wavelength directly, but you can, for example, measure the frequency and determine the wavelength from that.

On the other hand, if the conductor (transmission line) is much shorter than the wavelength of the signal, then distributed circuit effects are likely to be insignificant anyway, so the wavelength of the signal will likely not matter to the circuit's operation. For more clarification on this point, see this recent question.