Orthodoxy generally includes a common element (6)
I believe the answer is:
oxygen
'a common element' is the definition.
'oxygen' can be an answer for 'element' (oxygen is a kind of element). I am unsure of the 'a common' bit.
'orthodoxy generally includes' is the wordplay.
'includes' says the answer is hidden in the clue.
'OXYGEN' is hidden in the letters of 'orthodoxy generally'.
Can you help me to learn more?
(Other definitions for oxygen that I've seen before include "Gas that combines with hydrogen to form water" , "Gas (of publicity?)" , "life support" , "Gas forming about 20 per cent of atmosphere" , "Atmospheric gas" .)