Trek with donkey covered by sheet (7)
I believe the answer is:
passage
'trek' is the definition.
(thesaurus)
'donkey covered by sheet' is the wordplay.
'donkey' becomes 'ass' (I've seen this before).
'covered by' is an insertion indicator.
'sheet' becomes 'page' (page is a kind of sheet).
'ass' put within 'page' is 'PASSAGE'.
'with' acts as a link.
(Other definitions for passage that I've seen before include "Sea or air journey" , "Excerpt" , "Short section of text" , "Alley - corridor" , "street" .)