Crossword Genius

Defended only with locks (10)

Ross

I believe the answer is:

buttressed

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'defended' is the definition.
Although both the answer and definition are past participle verbs, I can't see how they can define each other.

'only with locks' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'with locks' becomes 'tressed' (locks and tresses can both mean hair).
'but'+'tressed'='BUTTRESSED'

Can you help me to learn more?

(Other definitions for buttressed that I've seen before include "Supported" , "held securely" , "Reinforced" .)

I've seen this clue in The Independent.
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