Saw dog restrained by lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'dog restrained by lead' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'restrained by' indicates putting letters inside.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' going inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Well-known statement incorporating general truth" , "Maxim" , "Pithy saying expressing a general truth" , "Adage, pithy saying" , "encapsulate wisdom" .)