Popular aquarium fish (5)

I think it could be either:

Which length description are you looking for?:

tetra

'popular aquarium fish' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for tetra that I've seen before include "... Four < , in composition" , "rarely seen flower" , "Wrongful act (Law)" , "Small tropical fish" , "Bright tropical fish" .)

I've seen this clue in The Times.

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