Underside of the foot (4)
I believe the answer is:
sole
'underside of the foot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sole that I've seen before include "Lone; part of foot" , "fish > that's never less than foot in length" , "Kind of fish - lose it" , "Fish; part of foot" , "piece of leather" .)