Thin and weak-looking, unlike Rose (5)
I believe the answer is:
weedy
'thin' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for weedy that I've seen before include "scrawny" , "Underdeveloped" , "Thin, puny (colloq.)" , "like an untended flower bed?" , "Skinny" .)