It's very easy to scratch head and part of foot (4)
I believe the answer is:
inch
'part of foot' is the definition.
('inch' is part of 'foot')
'it's very easy to scratch head' is the wordplay.
'it's very easy' becomes 'cinch' (informal term for an easy task).
'to scratch head' means to remove the first letter (I've seen 'scratching head' mean this).
'cinch' with its initial letter taken away is 'INCH'.
'and' is the link.
(Other definitions for inch that I've seen before include "Rare bird follower" , "Part of a foot" , "Imperial unit of length" , "Edge towards" , "0.08333 of a foot" .)