Saw dog being restrained by lead (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'dog being restrained by lead' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'being restrained by' is an insertion indicator.
'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 "Saying of great truth" , "Make haste slowly, perhaps" , "'Adage, saw (7)'" , "Familiar saying" , "Traditional saying" .)