Helpful to have the real reason for Fawkes's failure? (2,3)

I believe the answer is:

of use

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

'helpful to have' is the definition.
(I've seen this in another clue)

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

(Other definitions for of use that I've seen before include "Handy" , "practical" , "Advantageous" , "Useful" , "Nifty" .)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app