In precarious health like Bede, needing fuel regularly to replace some energy (10)

I believe the answer is:

vulnerable

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'in' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for vulnerable that I've seen before include "Susceptible" , "Exposed" .)

I've seen this clue in The Times.

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