Test a plexiglass housing key element (6)

I believe the answer is:

staple

'key element' is the definition.
(as in a staple of one's diet)

'test a plexiglass housing' is the wordplay.
'housing' says the answer is hidden in the clue (part of the clue houses the answer).
'STAPLE' is hidden in the letters of 'test a plexiglass'.

(Other definitions for staple that I've seen before include "e.g. rice, sweetcorn ..." , "Clip" , "Paper pin - basic" , "Paper-fastening device" , "Main, important" .)

I've seen this clue in The Guardian.

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