Mutual relationship in science (9)
I believe the answer is:
chemistry
'science' is the definition.
(I know that chemistry is a type of natural science)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for chemistry that I've seen before include "The science of matter" , "Science - mutual attraction" , "Scientific study of substances - by pharmacist?" , "A science (of personal attraction?)" , "scientific discipline" .)