Climber, one going over sides of valley (3)
I believe the answer is:
ivy
'climber' is the definition.
(thesaurus)
'one going over sides of valley' is the wordplay.
'one' becomes 'i' (Roman numeral).
'going over' is a charade indicator (letters next to each other).
'sides of' suggests removing the centre (only the letters on the sides remain).
'valley' with its centre removed is 'vy'.
'i'+'vy'='IVY'
(Other definitions for ivy that I've seen before include "With holly, it's seasonally decorative" , "The ... League comprises USA's top universities" , "Hedera helix" , "Girl" , "Parasitical plant, companion to holly" .)