Highlight possible cause or effect of illness (6)

I believe the answer is:

stress

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'highlight' is the definition.
(I know that highlight is a more specific form of stress)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for stress that I've seen before include "what causes tension" , "Dwell on" , "Underline - tension" , "Emphasis on anxiety?" , "Particular emphasis" .)

I've seen this clue in The Guardian.

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