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