Crossword Genius

Attack head demonstrating evolutionary survival principle (7)

Ross

I believe the answer is:

fitness

I'm a little stuck... Click here to teach me more about this clue!

'evolutionary survival principle' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'attack head' is the wordplay.
'attack' becomes 'fit' (fit is a kind of attack).
'head' becomes 'ness' (ness is a headland).
'fit'+'ness'='FITNESS'

'demonstrating' acts as a link.

Can you help me to learn more?

(Other definitions for fitness that I've seen before include "State of good health" , "Suitability - health" , "Competence" , "condition" , "Well-being" .)

I've seen this clue in The Guardian.
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