Designer of scale model of car better off without it (7)
I believe the answer is:
richter
'designer of scale' is the definition.
(as in the Richter scale)
'model of car better off without it' is the wordplay.
'model of car' becomes 'T' (Ford Model T).
'better off' becomes 'richer' (better off means with more money).
'without it' indicates putting letters inside.
't' put within 'richer' is 'RICHTER'.
(Other definitions for richter that I've seen before include "Scale on which earth tremors are measured" , "This scale measures the strength of earthquakes" , "Physicist" , "This scale measures earth tremors" , "Earthquake scale" .)