Saying that wanderer is in the lead (7)
I believe the answer is:
proverb
'saying' is the definition.
(I know that proverb is a type of saying)
'wanderer is in the lead' is the wordplay.
'wanderer' becomes 'rover' (synonyms).
'is in the' is an insertion indicator.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' going into 'pb' is 'PROVERB'.
'that' is the link.
(Other definitions for proverb that I've seen before include "Familiar saying" , "Make haste slowly, perhaps" , "OT saying" , "Generally true saying" , "Saying of great truth" .)