500 helpers for Santa mostly in burrow (5)

I believe the answer is:

delve

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

'burrow' is the definition.
(I know that burrow is a more specific form of the action delve)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for delve that I've seen before include "Rummage deep (in)" , "Search deeply for information" , "Turn up - dig" , "Dig - research deeply" , "Dig deep" .)

I've seen this clue in The Telegraph.

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