Footwear in sack (4)
I believe the answer is:
boot
This clue is a double definition.
'footwear' is the first definition.
(boot is a kind of footwear)
'sack' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for boot that I've seen before include "Luggage compartment" , "Dismiss" , "Sturdy shoe; start (a computer)" , "Kick; footwear" , "eg Wellington" .)