Saw king finished in lead (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'king finished in lead' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'in' indicates putting letters inside.
'lead' becomes 'pb' (Pb is the chemical symbol for lead).
'r'+'over'='rover'
'rover' going into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "'Adage, saw (7)'" , "Well-known statement incorporating general truth" , "Adage, pithy saying" , "encapsulate wisdom" , "Traditional saying" .)