Article on gold, with variable speculation (6)
I believe the answer is:
theory
'speculation' is the definition.
(something theorised or speculated)
'article on gold with variable' is the wordplay.
'article' becomes 'the' (the definite article in English).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'with' is a charade indicator (letters next to each other).
'variable' becomes 'y' (algebra).
'the'+'or'+'y'='THEORY'
(Other definitions for theory that I've seen before include "Opinion or supposition to explain something" , "Explanation of a phenomenon" , "Principle on which practice is based" , "General principle which explains some aspect of the world" , "Series of ideas seeking to explain something" .)