Flips out about foot joint (6)
I believe the answer is:
spliff
'joint' is the definition.
(both can mean a cannabis cigarette)
'flips out about foot' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'about' means one lot of letters goes inside another.
'foot' becomes 'f' (abbreviation).
'flips' anagrammed gives 'splif'.
'splif' enclosing 'f' is 'SPLIFF'.
(Other definitions for spliff that I've seen before include "drug dose" , "Pot cigarette (colloq.)" , "Cannabis joint" , "Blow in this tube" , "Marijuana cigarette" .)