Climber starting off is very young (3)
I believe the answer is:
ivy
'climber' is the definition.
(thesaurus)
'starting off is very young' is the wordplay.
'starting off' says to take the initial letters.
The first letters of 'is very young' is 'IVY'.
(Other definitions for ivy that I've seen before include "Parasitical plant, companion to holly" , "Female" , "Four-year" , "Bush" , "Old World vine" .)