Climber last in trek reaching summit (4)
I believe the answer is:
peak
'summit' is the definition.
(mountain peak)
'climber last in trek' is the wordplay.
'climber' becomes 'pea' (the pea plant is a climbing plant).
'last in' suggests the final letters.
The last letter of 'trek' is 'k'.
'pea'+'k'='PEAK'
'reaching' is the link.
(Other definitions for peak that I've seen before include "Sharp pointed summit" , "Summit, often pointed" , "Reach the highest point" , "Tip" , "Pinnacle" .)