Saw car in lead (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'car in lead' is the wordplay.
'car' becomes 'rover' (former British car manufacturer**).
'in' means one lot of letters goes inside another.
'lead' becomes 'pb' ('Pb' can be a synonym of 'lead').
'rover' put into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "'Adage, saw (7)'" , "Familiar saying" , "Saying of great truth" , "Short saying expressing a supposed truth" , "Aphorism" .)