Saw senior scout formerly in lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'senior scout formerly in lead' is the wordplay.
'senior scout formerly' becomes 'rover' (Rover Scouts is a Boy Scouts organisation).
'in' indicates putting letters inside.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' inserted within 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Traditional saying" , "gnome" , "Make haste slowly, perhaps" , "Maxim" , "Aphorism" .)