Saying pirate goes into lead (7)
I believe the answer is:
proverb
'saying' is the definition.
(I know that saying can be written as proverb)
'pirate goes into lead' is the wordplay.
'pirate' becomes 'rover' (I've seen this before).
'goes into' means one lot of letters goes inside another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' going inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Maxim" , "Short saying embodying a supposed truth" , "Old saying, perhaps by Solomon?" , "Pithy saying expressing a general truth" , "Well-known statement incorporating general truth" .)