Plant holds second beast up (7)
I believe the answer is:
oregano
'plant' is the definition.
(oregano is a kind of plant)
'holds second beast up' is the wordplay.
'holds second' becomes 'o' (2nd letter of 'holds').
'beast' becomes 'onager' (equine species native to Asia).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'onager' written backwards gives 'regano'.
'o'+'regano'='OREGANO'
(Other definitions for oregano that I've seen before include "Sweet-smelling herb" , "Kitchen herb" , "Mediterranean herb" , "Herb related to marjoram" , "Aromatic culinary herb" .)