Tabitha changes environment (7)
I believe the answer is:
habitat
'environment' is the definition.
(habitat is a kind of environment)
'tabitha changes' is the wordplay.
'changes' indicates an anagram.
'tabitha' with letters rearranged gives 'HABITAT'.
(Other definitions for habitat that I've seen before include "One's normal environment" , "Natural environment, plant or animal" , "(Organism's) natural home" , "Environment in which an organism normally lives" , "Territory" .)