Teaches beginners to trek up hills (6)
I believe the answer is:
tutors
'teaches' is the definition.
(tutoring is a kind of teaching)
'beginners to trek up hills' is the wordplay.
'beginners' indicates taking the first letters.
'hills' becomes 'tors' (tor is a kind of hill).
The first letters of 'trek up' is 'tu'.
'tu'+'tors'='TUTORS'
(Other definitions for tutors that I've seen before include "Teaches" , "Private teachers for students" , "Teachers of individuals or small groups" , "Private instructors" , "Mentors" .)