I must infiltrate merchants heading off marauders (7)

I believe the answer is:

raiders

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

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

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

(Other definitions for raiders that I've seen before include "' - - of the Lost Ark'" , "Marauders" , "Attacking force" , "Marauding attackers" .)

I've seen this clue in The Independent.

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