Defended only with locks (10)
I believe the answer is:
buttressed
'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" .)