Opposed to admitting disorderly bar element (6)
I believe the answer is:
carbon
'element' is the definition.
(I know that carbon is a type of element)
'opposed to admitting disorderly bar' is the wordplay.
'opposed to' becomes 'con' (I've seen this in another clue).
'admitting' indicates putting letters inside (admit can mean to welcome in or accept).
'disorderly' indicates an anagram.
'bar' anagrammed gives 'arb'.
'con' placed around 'arb' is 'CARBON'.
(Other definitions for carbon that I've seen before include "Dioxide or monoxide, say" , "Common element" , "Chemical element occurring in pure form in diamond" , "C (with a footprint)" , "Non-metallic element, C" .)