What is the grammatical function of this 'action' in this sentence?

I read a sentence from the economist

Fed also propped up the corporate-bond market, action it had shied away from during the financial crisis a decade earlier.

Although I know the meaning of the sentence,I can’t understand the grammatical function of the ‘action’.At first, I thought it was an Appositive or Complement.Then I realized that appositive and complement can only supplement or replace the noun component, but can not explain the predicate component.In meaning, the action refers to the preceding predicate action——propping up.But as far as I am concerned, no word appears for no reason and always needs to be used as a grammatical element. It can only be subject, predicate, object, adverbial, attribute or appositive.So I hope you can help me to explain the grammatical function of this word action.Thank you!