Unusually active when it retreats to grotto (4)

I believe the answer is:

cave

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

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

(Other definitions for cave that I've seen before include "Natural underground space" , "Warning - grotto" , "Hollow in a cliff or underground" , "Troglodyte's home" , "Underground enclosure with surface access" .)

I've seen this clue in The Hindu.

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