Fundamental quality of physical life (6)
I believe the answer is:
nature
'fundamental quality' is the definition.
'nature' can be an answer for 'quality' (nature is a kind of quality). I am not sure about the 'fundamental' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nature that I've seen before include "Innate quality" , "It abhors a vacuum, according to Aristotle" , "- Study; - reserve" , "Fundamental qualities" , "The external, untouched world" .)