It's just under a foot (6)
'it's just under a foot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for insole that I've seen before include "Boots may be holding it" , "Inner part of shoe" , "Padding in a shoe" , "found in boot" , "Lesion in part of foot" .)
(Other definitions for sole that I've seen before include "piece of leather" , "Dover - -" , "fish > that's never less than foot in length" , "Bottom of the foot" , "Flatfish; exclusive" .)