Posh in flowing garment causes debate (8)
I believe the answer is:
argument
'debate' is the definition.
(I know that debate can be written as argument)
'posh in flowing garment' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'in' means one lot of letters goes inside another.
'flowing' is an anagram indicator.
'garment' is an anagram of 'argment'.
'u' placed within 'argment' is 'ARGUMENT'.
'causes' acts as a link.
(Other definitions for argument that I've seen before include "'Debate, row (8)'" , "Heated disagreement" , "Altercation" , "Debate - disagreement" , "One or more reasons" .)