Plant elm, pine, park oddly in a muddle (9)
I believe the answer is:
pimpernel
'plant' is the definition.
(pimpernel is a kind of plant)
'elm pine park oddly in a muddle' is the wordplay.
'oddly' indicates alternate letters.
'in a muddle' indicates an anagram.
The alternating letters of 'park' are 'pr'.
'elm'+'pine'+'pr'='elmpinepr'
'elmpinepr' anagrammed gives 'PIMPERNEL'.
(Other definitions for pimpernel that I've seen before include "(Scarlet) plant of the primrose family" , "Low-growing plant" , "Small scarlet flower" , "garden plant" , "Baroness Orczy's scarlet flower?" .)