Scoundrel lost head for a great swimmer (5)
I believe the answer is:
otter
'swimmer' is the definition.
(otters are animals which swim)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for otter that I've seen before include "carnivore is concerned" , "Lithe swimmer" , "Water creature" , "River creature" , "Semi-aquatic mammal" .)