Still locks supporting structures (10)

I believe the answer is:

buttresses

'supporting structures' is the definition.
(buttress is a kind of supporting structure)

'still locks' is the wordplay.
'still' becomes 'but' (I've seen this in other clues).
'locks' becomes 'tresses' (I've seen this before).
'but'+'tresses'='BUTTRESSES'

(Another definition for buttresses that I've seen is " parts of building".)

I've seen this clue in the Sydney Morning Herald.

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