Cross states in endless trek (8)
I believe the answer is:
traverse
'cross' is the definition.
(synonyms)
'states in endless trek' is the wordplay.
'states' becomes 'avers' (I've seen this before).
'in' indicates putting letters inside.
'endless' means to remove the last letter.
'trek' with its final letter removed is 'tre'.
'avers' inserted into 'tre' is 'TRAVERSE'.
(Other definitions for traverse that I've seen before include "Ski diagonally across (a slope)" , "In mountaineering, a steep slope to be crossed from side to side" , "Move across" , "Go across" , "get across" .)