Distance covered by some trekkers (5)
I believe the answer is:
metre
'distance' is the definition.
(unit of distance)
'covered by some trekkers' is the wordplay.
'covered by' indicates the answer is hidden within the clue.
'METRE' is hidden within 'some trekkers'.
(Other definitions for metre that I've seen before include "measure of length" , "Moisture remover" , "Beat pattern (mus.)" , "Basic unit of length" , "Approx 39 inches" .)