The government is seeking to take out things from the political declaration that Theresa May spent time getting in.
What does getting in mean?
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... that Theresa May spent time getting in [the political declaration].
Theresa May was the leader of the Conservative Party and the Prime Minister of UK before being succeeded by Boris Johnson.
During the time Ms. May was in power, she "expended huge amounts of political capital" putting some things (e.g., trade deal promises or regulations - see the video) in the political declaration. The new leader, Boris Johnson, is planning to take some of those provisions out of the political declaration.
In short, Boris Johnson is planning to remove some of the regulations/laws that Theresa May had previously put in.
Unable to find his car keys, John took out all of the folded laundry from the wardrobe that Jane had spent hours putting in.
This means that Jane had spent a lot of time putting folded laundry in the cabinet.