Cases crack in middle of sled (7)
I believe the answer is:
luggage
'cases' is the definition.
(I've seen this before)
'crack in middle of sled' is the wordplay.
'crack' becomes 'gag' (both can mean a joke).
'in middle of' indicates putting letters inside.
'sled' becomes 'luge' (luging is a kind of sledding).
'gag' placed into 'luge' is 'LUGGAGE'.
(Other definitions for luggage that I've seen before include "Impedimenta" , "case?" , "Bags of the traveller" , "Travelling bags etc." , "Suitcases and travelling bags" .)