Italicize or underline (6)
I believe the answer is:
stress
I believe this is a double definition.
'italicize' is the first definition.
(I've seen this in another clue)
'underline' is the second definition.
(I know that underline is a more specific form of the action stress)
(Other definitions for stress that I've seen before include "Put weight on" , "what causes tension" , "why off work?" , "Draw attention to" , "nervous tension" .)