Mostly courteous hanger-on joining old Communist committee (9)
I believe the answer is:
politburo
'communist committee' is the definition.
(type of committee in communist states)
'mostly courteous hanger-on joining old' is the wordplay.
'mostly' means to remove the last letter.
'courteous' becomes 'polite' (similar in meaning).
'hanger-on' becomes 'bur' (type of seed which hangs onto clothing).
'joining old' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'polite' with its final letter taken away is 'polit'.
'polit'+'bur'+'o'='POLITBURO'
(Other definitions for politburo that I've seen before include "executive committee" , "Commie leaders" , "Ruling Soviet committee" , "Former USSR top committee" , "Former USSR rulers" .)