Underline “anxiety” (6)
I believe the answer is:
stress
I believe this clue is a double definition.
'underline' is the first definition.
(I know that stress is a is a more specific form of the action of underline)
'anxiety' is the second definition.
(anxiety is stress or tension)
(Other definitions for stress that I've seen before include "possible cause of fracture?" , "Pressure or strain" , "Lay emphasis on - it's a strain" , "suffering" , "Emphasise the strain" .)