Old rodent climbing plant (4)
I believe the answer is:
taro
'plant' is the definition.
(taro is a kind of plant)
'old rodent climbing' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rodent' becomes 'rat' (rat is a kind of rodent).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'o'+'rat'='orat'
'orat' written backwards gives 'TARO'.
(Other definitions for taro that I've seen before include "Tropical plant with edible tuberous roots" , "Starchy plant" , "Starchy food" , "Fortune-telling cards" , "A rot turns the lily-like plant" .)