Relative speed of light follows Newton’s first — that is incontrovertible, ultimately (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'speed of light follows newton's first that is incontrovertible ultimately' is the wordplay.
'speed of light' becomes 'c' (I've seen this in other clues).
'follows' says to put letters next to each other.
'newton's first' becomes 'n' (1st letter of 'newton').
'that is' becomes 'i.e.' (synonyms).
'ultimately' says to take the final letters.
The final letter of 'incontrovertible' is 'e'.
'n'+'ie'='nie'
'c' put after 'nie' is 'niec'.
'niec'+'e'='NIECE'
(Other definitions for niece that I've seen before include "Family relative" , "Young female relative" , "Criminal" , "One of the family" , "Son's cousin" .)