Pile up flotsam as shipwreck submerges (5)

I believe the answer is:

amass

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'pile up' is the definition.
(synonyms)

'flotsam as shipwreck submerges' is the wordplay.
'AMASS' is hidden within the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for amass that I've seen before include "Collect, hoard" , "Accumulate, pile up" , "store" , "Get together" , "Gather together, pile up" .)

I've seen this clue in The Independent.

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