Craze that is, we're told, increasingly 'leonine'? (5)
I believe the answer is:
'craze' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mania that I've seen before include "Insanity" , "Madness - craze" , "Obsessional enthusiasm" , "serious obsession" , "wildness" .)