They go under water and come back glossy (5)

I believe the answer is:

keels

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'they go under water' is the definition.
The definition suggests an adverb but the answer is not.

'come back glossy' is the wordplay.
'come back' is a reversal indicator.
'glossy' becomes 'sleek' (synonyms).
'sleek' back-to-front is 'KEELS'.

'and' acts as a link.

Can you help me to learn more?

(Other definitions for keels that I've seen before include "Parts of ships" , "bases of vessels" .)

I've seen this clue in The Globe and Mail.

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