Drove Mustang, say, when Ford finally replaced Sierra (5)

I believe the answer is:

horde

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'drove' is the definition.
(synonyms)

I don't understand the remainder of the clue.

(Other definitions for horde that I've seen before include "A swarm or multitude" , "A moving crowd" , "Large crowd" , "Great crowd" , "Large army" .)

I've seen this clue in The Guardian.

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