Rhett's confused about Newton and gravity's force (8)
I believe the answer is:
strength
'force' is the definition.
(I know that force can be written as strength)
'rhett's confused about newton and gravity's' is the wordplay.
'confused' is an anagram indicator.
'about' means one lot of letters goes inside another.
'newton' becomes 'N' (symbol for the scientific unit of force).
'and' means one lot of letters go next to another.
'gravity' becomes 'g'.
'rhetts' with letters rearranged gives 'streth'.
'n'+'g'='ng'
'streth' going around 'ng' is 'STRENGTH'.
(Other definitions for strength that I've seen before include "Good attribute" , "Intensity" , "Vigour, potency" , "Physical or mental capacity" , "A good quality; might" .)