Joint almost broken with doubled force (6)
I believe the answer is:
spliff
'joint' is the definition.
(both can mean a cannabis cigarette)
'almost broken with doubled force' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'broken' becomes 'split' (similar in meaning).
'with doubled force' becomes 'ff' (I can't justify this - if you can you should believe this answer much more).
'split' with its last letter taken off is 'spli'.
'spli'+'ff'='SPLIFF'
(Other definitions for spliff that I've seen before include "Blow in this tube" , "Cannabis joint" , "Pot cigarette (colloq.)" , "Marijuana fag" , "Dodgy smoke" .)