Top of Bowfell, on ascending after morning light (5)
I believe the answer is:
amber
'light' is the definition.
(I've seen this before)
'top of bowfell on ascending after morning' is the wordplay.
'top of' indicates taking the first letters.
'on' becomes 're' ('on' can be mean 'with reference to').
'ascending' says the letters should be written backwards.
'after' means one lot of letters go next to another.
'morning' becomes 'am' (as in 12:00 AM).
The initial letter of 'bowfell' is 'b'.
're' written backwards gives 'er'.
'b'+'er'='ber'
'ber' after 'am' is 'AMBER'.
(Other definitions for amber that I've seen before include "Orange traffic light" , "Fossil resin - bream (anag)" , "Colour of warning" , "Young bird of prey" , "Fossil resin - warning light" .)