Barney Rubble's opening a Nutmeg cryptic (8)
I believe the answer is:
argument
'barney' is the definition.
(I've seen this before)
'rubble's opening a nutmeg cryptic' is the wordplay.
'opening' indicates taking the first letters.
'cryptic' indicates anagramming the letters (a cryptic or obscure order of the letters).
The initial letter of 'rubbles' is 'r'.
'r'+'a'+'nutmeg'='ranutmeg'
'ranutmeg' is an anagram of 'ARGUMENT'.
(Other definitions for argument that I've seen before include "Opposed points of view" , "Heated exchange" , "'Debate, row (8)'" , "Heated debate" , "Line of reasoning" .)