Saying rambler is in lead (7)
I believe the answer is:
proverb
'saying' is the definition.
(I know that proverb is a type of saying)
'rambler is in lead' is the wordplay.
'rambler' becomes 'rover' (I've seen this before).
'is in' means one lot of letters goes inside another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' put inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Old saying, perhaps by Solomon?" , "Short saying embodying a supposed truth" , "Maxim" , "Adage, pithy saying" , "'Adage, saw (7)'" .)