Pet subject for experimental scientist (6,3)
I believe the answer is:
guinea pig
'pet' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for guinea pig that I've seen before include "one used for experimentation" , "First to use" , "Domestic pet - experimental victim" , "One experimented upon" , "rodent eaten in Peru" .)