Saw dog held by lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'dog held by lead' is the wordplay.
'dog' becomes 'rover' (Rover is a common name for a pet dog).
'held by' means one lot of letters goes inside another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' inserted inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "'Adage, saw (7)'" , "Saying encapsulating a general truth" , "Saying of great truth" , "Old saw" , "Generally true saying" .)