Saw car going into lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'car going into lead' is the wordplay.
'car' becomes 'rover' (former British car manufacturer).
'going into' is an insertion indicator.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' put within 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "OT saying" , "Aphorism" , "Short saying expressing a supposed truth" , "gnome" , "encapsulate wisdom" .)