Crossword Genius

What a spider does is highly precarious (5,2,1,6)

Ross

I believe the answer is:

hangs by a thread

'is highly precarious' is the definition.
(to hang by a thread is to be in a precarious state)

'what a spider' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'does' is the link.

I've seen this clue in The Times.
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