Walker may use this trick to take in silly sap (8)
I believe the answer is:
knapsack
'walker may use this' is the definition.
(I know this)
'trick to take in silly sap' is the wordplay.
'trick' becomes 'knack' (as in having the knack for something).
'to take in' means one lot of letters goes inside another.
'silly' indicates an anagram.
'sap' with letters rearranged gives 'aps'.
'knack' going around 'aps' is 'KNAPSACK'.
(Other definitions for knapsack that I've seen before include "(Hiker's) backpack" , "Bag with straps" , "Traveller's or soldier's bag" , "Hiking bag" , "Bag carried on back or shoulder" .)