Tailless tarantula is bothering botanist (10)
I believe the answer is:
naturalist
'botanist' is the definition.
(thesaurus)
'tailless tarantula is bothering' is the wordplay.
'tailless' means to remove the last letter.
'bothering' indicates anagramming the letters.
'tarantula' with its final letter taken off is 'tarantul'.
'tarantul'+'is'='tarantulis'
'tarantulis' anagrammed gives 'NATURALIST'.
(Other definitions for naturalist that I've seen before include "Darwin?" , "Zoologist" , "One who studies animals and plants" , "one's studying in the wild" , "Zoology expert possibly" .)