Furry aquatic creature has wizard scratching head (5)
I believe the answer is:
otter
'furry aquatic creature' is the definition.
(otters are furry)
'wizard scratching head' is the wordplay.
'wizard' becomes 'potter' (Harry Potter).
'scratching head' means to remove the first letter (the word's head is removed).
'potter' with its initial letter taken off is 'OTTER'.
'has' is the link.
(Other definitions for otter that I've seen before include "Lithe swimmer" , "River animal" , "Riverine mammal" , "Water-loving animal" , "carnivore is concerned" .)