Cautiously move part of hand or foot (4)
I believe the answer is:
inch
'cautiously move' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'part of hand or foot' is the wordplay.
'part of' becomes 'in' (being in something means being part of it).
'hand or foot' becomes 'ch' (I can't justify this - if you can you should believe this answer much more).
'in'+'ch'='INCH'
Can you help me to learn more?
(Other definitions for inch that I've seen before include "Measure of length" , "Two point five four cm" , "Edge slowly" , "Edge towards" , "Move along slowly" .)