New garment you must put on Barney (8)
I believe the answer is:
argument
'barney' is the definition.
(I've seen this before)
'new garment you must put on' is the wordplay.
'new' is an anagram indicator.
'you' becomes 'u' (commonly used in online communication).
'must put on' means one lot of letters goes inside another (inserted letters put on or 'wear' others).
'garment' anagrammed gives 'argment'.
'argment' placed around 'u' is 'ARGUMENT'.
(Other definitions for argument that I've seen before include "Barney" , "that may be constructed on premises" , "Clash of opinions" , "Contentious debate" , "Tiff" .)