Knock-kneed fullback far from totally indispensable (7)

I believe the answer is:

needful

I'm a little stuck... Click here to teach me more about this clue!

'indispensable' is the definition.
(I've seen this before)

'knock-kneed fullback far from totally' is the wordplay.
'NEEDFUL' is hidden within the letters.
But, I am not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for needful that I've seen before include "Requisite, necessary" , "Required" , "Nude elf (anag.)" , "poor" , "Necessary for nude elf" .)

I've seen this clue in The Guardian.

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