Traveller climbing headland shown in electronic device (9)
I believe the answer is:
passenger
'traveller' is the definition.
(I know that Passenger is a type of passenger)
'climbing headland shown in electronic device' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'headland' becomes 'ness' (both can mean a promontory).
'shown in' indicates putting letters inside.
'electronic device' becomes 'pager' (I've seen this before).
'ness' back-to-front is 'ssen'.
'ssen' put inside 'pager' is 'PASSENGER'.
(Other definitions for passenger that I've seen before include "One on a train perhaps" , "Fare-paying traveller" , "Traveller not driving" , "not contributing to other people's efforts?" , "One travelling" .)