Subject of Newton's first law (7)
I believe the answer is:
inertia
'subject of newton's first law' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inertia that I've seen before include "Unwillingness to move" , "Tendency to inaction" , "Admonish" , "Chemical or other inactivity" , "Lethargy" .)