Kitchen heater tested in pair's absence (4)

I believe the answer is:

oven

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'kitchen heater' is the definition.
'oven' can be an answer for 'heater' (thesaurus). I'm unsure of the 'kitchen' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for oven that I've seen before include "You can cook in this" , "Section of a cooker" , "kitchen fixture" , "Part of a cooker" , "Stove" .)

I've seen this clue in The Sun.

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