Perennial lover of lassie hidden in long grass (8)

I believe the answer is:
marjoram
'perennial lover' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'lassie hidden in long grass' is the wordplay.
'lassie' becomes 'Jo'.
'hidden in' indicates putting letters inside.
'long grass' becomes 'marram' (marram is a type of grass. I am not sure about the 'long' bit.).
'jo' placed into 'marram' is 'MARJORAM'.
'of' is the link.
Can you help me to learn more?
(Other definitions for marjoram that I've seen before include "Arm major (anag)" , "Herb used in cooking - a Mr Major (anag)" , "Herb, maybe oregano" , "Aromatic herb" , "Herb used in soups, stews etc" .)
