Stress over work? Quite the opposite (9)
I believe the answer is:
underline
'stress' is the definition.
(I know that underline is a more specific form of the action stress)
'over work? quite the opposite' is the wordplay.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'work?' becomes 'line' (line can mean someone's line of work).
'quite the opposite' becomes 'under' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'line' put after 'under' is 'UNDERLINE'.
Can you help me to learn more?
(Other definitions for underline that I've seen before include "Method of showing emphasis in writing" , "To show emphasis in writing" , "Reinforce" , "Emphasise with horizontal drawing" , "Call attention to - highlight" .)