Plant, mostly forgotten, occupied by organised labour (5)
I believe the answer is:
lotus
'plant' is the definition.
(lotus is a flowering plant)
'mostly forgotten occupied by organised labour' is the wordplay.
'mostly' means to remove the last letter.
'forgotten' becomes 'lost' (similar in meaning).
'occupied by' means one lot of letters goes inside another.
'organised labour' becomes 'tu' (I can't justify this - if you can you should believe this answer much more).
'lost' with its final letter taken away is 'los'.
'los' going around 'tu' is 'LOTUS'.
Can you help me to learn more?
(Other definitions for lotus that I've seen before include "F1 team and car maker" , "Plant widely regarded as sacred" , "Type of water lily" , "Plant - yoga position" , "Sacred Egyptian water lily" .)