A form of weed inside thin, drained plant (4,4)
I believe the answer is:
tree fern
'plant' is the definition.
(tree fern is a kind of plant)
'a form of weed inside thin drained' is the wordplay.
'a form of weed' becomes 'reefer' (I can't explain this - if you can you should believe this answer much more).
'inside' indicates putting letters inside.
'drained' means to remove the middle letters.
'thin' with its middle taken out is 'tn'.
'reefer' inserted inside 'tn' is 'TREE-FERN'.
Can you help me to learn more?
(Other definitions for tree fern that I've seen before include "Palm-like plant" , "Tall plant" , "tropical plant" .)