Naturalist appearing in calendar — WI, naturally! (6)
I believe the answer is:
darwin
'naturalist' is the definition.
(Darwin is an example)
'in calendar wi naturally' is the wordplay.
'in' says the answer is hidden in the clue.
'DARWIN' is hidden within 'calendar wi naturally'.
'appearing' acts as a link.
(Other definitions for darwin that I've seen before include "He proposed the theory of evolution" , "Natural scientist and Australian city" , "Charles ...... originated the theory of evolution" , "Australia's northernmost state capital" , "Naturalist with a theory on evolution" .)