Steed provides mobile assistance (7)
I believe the answer is:
charger
'steed' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for charger that I've seen before include "Horse - plate" , "Type of platter" , "Horse; flat dish" , "One loading" , "What Richard III wanted, perhaps" .)