Plant we liked, wild under top of mountain (8)

'plant' is the definition.
(milkweed is a kind of plant)

'we liked wild under top of mountain' is the wordplay.
'wild' is an anagram indicator.
'under' says to put letters next to each other.
'top of' says to take the initial letters.
The first letter of 'mountain' is 'm'.
'we'+'liked'='weliked'
'weliked' anagrammed gives 'ilkweed'.
'ilkweed' put after 'm' is 'MILKWEED'.