Climber, one into obsessive climbing (5)
I believe the answer is:
liana
'climber' is the definition.
(climbing plant)
'one into obsessive climbing' is the wordplay.
'one' becomes 'I' (Roman numeral).
'into' means one lot of letters goes inside another.
'obsessive' becomes 'anal' (I've seen this before).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'i' inserted into 'anal' is 'anail'.
'anail' reversed gives 'LIANA'.
(Other definitions for liana that I've seen before include "Woody climbing (usually tropical) plant" , "Woody forest climber" , "climber in the tropics" , "Woody climbing tropical plant -- a nail (anag)" , "Climbing plant of rainforests" .)