Crossword Genius

Persistent trouble, tough, all after first round (5)

Ross

I believe the answer is:

hydra

'persistent trouble' is the definition.
(the mythological hydra could grow back its heads)

I can't explain the remainder of the clue.

(Other definitions for hydra that I've seen before include "Persistent trouble" , "Nine-headed monster of Greek myth" , "Nine-headed monster killed by Hercules" , "Mythological water-monster" , "Greek island" .)

I've seen this clue in The Independent.
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