Saw wanderer in lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'wanderer in lead' is the wordplay.
'wanderer' becomes 'rover' (synonyms).
'in' means one lot of letters goes inside another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' going within 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "OT saying" , "Pithy saying expressing a general truth" , "Traditional saying" , "Short saying expressing a supposed truth" , "Make haste slowly, perhaps" .)