Relative acceleration with new model Escort (8)
I believe the answer is:
ancestor
'relative' is the definition.
(ancestor is a kind of relative)
'acceleration with new model escort' is the wordplay.
'acceleration' becomes 'a' (symbol used in Physics e.g. f=ma).
'with' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'model' indicates an anagram.
'escort' is an anagram of 'cestor'.
'n' inserted inside 'cestor' is 'ncestor'.
'a'+'ncestor'='ANCESTOR'
(Other definitions for ancestor that I've seen before include "Adam was our first one" , "Old member of the family -no traces (anag)" , "Forefather" , "Enactors (anag)" , "Antecedent, forebear" .)