Crossword Genius

Emphasise old resistance is source of tension (8)

Ross

I believe the answer is:

stressor

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'tension' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'emphasise old resistance is source of' is the wordplay.
'emphasise' becomes 'stress' ('stress' can be a synonym of 'emphasize'**).
'is source of' indicates taking the first letters.
The first letters of 'old resistance' is 'or'.
'stress'+'or'='STRESSOR'

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I've seen this clue in The Telegraph.
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