So slight as to remain undetected by the senses (13)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
imperceptible
(Other definitions for imperceptible that I've seen before include "Not discernible by the senses" , "Unnoticeable" , "Too slight to be noticed" , "Too small to be noticed" , "Barely detectable" .)