Otherwise eels thrash about (4)

I believe the answer is:

else

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'otherwise' is the definition.
(synonyms)

'eels thrash about' is the wordplay.
'eels' can be anagrammed to 'ELSE'.
Though, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for else that I've seen before include "In addition; different" , "Besides, otherwise" , "Otherwise; different from" , "Besides; different" , "As an alternative" .)

I've seen this clue in The Mirror.

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