Part of the foot to move slowly (4)
I believe the answer is:
inch
This clue is a double definition.
'part of the foot' is the first definition.
('inch' is part of 'foot')
'to move slowly' is the second definition.
(to inch along is to move slowly)
(Other definitions for inch that I've seen before include "Scottish island: unit of length" , "Imperial measure" , "One 63,360th of a mile" , "Linear measure" , "2.54cm" .)