Emphasise a state of nervous tension (6)
I believe the answer is:
stress
I believe this is a triple definition!
'emphasise' is the first definition.
(I know that emphasise can be written as stress)
'a state' is the second definition.
(to stress is to state with emphasis)
'of' acts as a link.
'nervous tension' is the third definition.
(I've seen this before)
(Other definitions for stress that I've seen before include "Tension - accent" , "Force" , "what causes tension" , "Tension; emphasis" , "Emphasis: result often of being under pressure" .)