Long-distance travellers in camp seen relaxing (8)
I believe the answer is:
spacemen
'long-distance travellers' is the definition.
'camp seen relaxing' is the wordplay.
'relaxing' is an anagram indicator.
'camp'+'seen'='campseen'
'campseen' with letters rearranged gives 'SPACEMEN'.
'in' acts as a link.
(Other definitions for spacemen that I've seen before include "Astronauts" , "High fliers" , "they have a high trajectory" , "long-distance travellers?" , "Adventurous travellers" .)