One very exhausted climber (3)
I believe the answer is:
ivy
'climber' is the definition.
(ivy is a climbing plant)
'one very exhausted' is the wordplay.
'one' becomes 'i' (Roman numeral).
'exhausted' means to remove the middle letters (the word is emptied out or exhausted).
'very' with its middle taken out is 'vy'.
'i'+'vy'='IVY'
(Other definitions for ivy that I've seen before include "Girl" , "Climbing shrub; f. name" , "Parasitical plant, companion to holly" , "Song: 'Poison --'" , "Trailer" .)