A slippery customer, albeit young? (5)

I believe the answer is:

elver

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'young?' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for elver that I've seen before include "Slippery youngster" , "One may see one upriver" , "Young swimmer" , "Young migratory fish" , "A young 11" .)

I've seen this clue in the Evening Standard.

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