They illuminate flimsy joists (5,5)

I believe the answer is:

light beams

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'they illuminate' is the definition.
The definition suggests an adverb but the answer is not.

'flimsy joists' is the wordplay.
'flimsy' becomes 'light' (I've seen this before).
'joists' becomes 'beams' (I've seen this in another clue).
'light'+'beams'='LIGHT BEAMS'

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

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