Expedition’s endless voyage, covering a great distance (6)
I believe the answer is:
safari
'expedition's' is the definition.
(safari is a kind of expedition)
'endless voyage covering a great distance' is the wordplay.
'endless' means to remove the last letter.
'voyage' becomes 'sail' (sail is a kind of voyage).
'covering' indicates putting letters inside.
'a great distance' becomes 'far' (synonyms).
'sail' with its final letter taken away is 'sai'.
'sai' placed around 'far' is 'SAFARI'.
(Other definitions for safari that I've seen before include "A journey to see or hunt wild animals" , "Big game expedition" , "Hunting journey" , "North African capital" , "Overland journey by hunters" .)