Distance below the knee (4)
I believe the answer is:
foot
'distance' is the definition.
(I have seen 'Imperial distance' mean 'foot' so perhaps 'distance' could also mean 'foot')
'below the knee' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'foo' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for foot that I've seen before include "Unit of length" , "Distance to the end of a limb?" , "support" , "Metrical unit; pay (a bill)" , "Measure - part of body" .)