Clue: At home in a condition that is unstable (8)
Answer: ASTATINE
Explanation: Definition: that is unstable
IN in A STATE 
~~Like 12ac, another vague definintion that the setter gets away with due to the theme.~~
 
(Andrew has suggested it is At that is the definition, and that STATE is indicated by "condition that is unstable", so I may have been unfair to Gozo)
 
Astatine is an unstable radioactive element.