Item of footwear in sack (4)
I believe the answer is:
boot
This clue is a double definition.
'item of footwear' is the first definition.
(I've seen this before)
'sack' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for boot that I've seen before include "Stout shoe" , "Sturdy footwear" , "Sturdy shoe; start (a computer)" , "eg Wellington" , "may be packed for a journey" .)