Causes (4)
'causes' is the definition.
(I know this)
This is all the clue.
(Other definitions for reasons that I've seen before include "Thinks logically" , "Justifications or motives" , "Ness oar (anag.)" , "Rationalises" , "Period of reduced economic activity" .)
(Other definitions for whys that I've seen before include "explanations" , "Reasons" , "They partner "wherefores"" , "Causes underlying an action or situation" .)