Steed is the chap making out the invoices (7)

I believe the answer is:

charger

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'steed' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for charger that I've seen before include "Horse; flat dish" , "Battle horse" , "Horse - plate" , "could be of use to battery" , "that would prevent loss of power?" .)

I've seen this clue in the Evening Standard.

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