Be in no hurry to insulate the boiler (3)

I believe the answer is:

lag

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'insulate the boiler' is the definition.
'lag' can be an answer for 'insulate' (I've seen this before). I am unsure of the 'the boiler' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for lag that I've seen before include "Cover pipes" , "person inside" , "Provide protection" , "Fall behind; insulate" , "Loiter" .)

I've seen this clue in The Globe and Mail.

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