Saw pirate in lead (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'pirate in lead' is the wordplay.
'pirate' becomes 'rover' (I've seen this before).
'in' indicates putting letters inside.
'lead' becomes 'pb' (Pb is the chemical symbol for lead).
'rover' inserted inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Short saying expressing a supposed truth" , "Make haste slowly, perhaps" , "Traditional saying" , "OT saying" , "Maxim" .)