Teacher: “According to Newton’s second law, force affects rest” (6)
I believe the answer is:
master
'teacher' is the definition.
(I've seen this before)
'force affects rest' is the wordplay.
'force' becomes 'ma' (I can't justify this - if you can you should believe this answer much more).
'affects' is an anagram indicator (I've seen 'affected' mean this).
'rest' with letters rearranged gives 'ster'.
'ma'+'ster'='MASTER'
'according to newton's second law' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for master that I've seen before include "Artist of consummate skill" , "Main; overcome; original recording" , "One has consummate skill" , "Subdue, win over" , "with 1a [NEW] Jacky may have in nursery" .)