Dog secured by lead — better late than never? (7)

I believe the answer is:
proverb
'better late than never?' is the definition.
I can't judge whether this defines the answer.
'dog secured by lead' is the wordplay.
'dog' becomes 'rover' (Rover is a common name for a pet dog).
'secured by' means one lot of letters goes inside another (inserted letters are held or secured).
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' put inside 'pb' is 'PROVERB'.
Can you help me to learn more?
(Other definitions for proverb that I've seen before include "Short saying embodying a supposed truth" , "Well-known statement incorporating general truth" , "Saying encapsulating a general truth" , "Old saw" , "'Adage, saw (7)'" .)
