Saying when pet dog wears lead (7)
I believe the answer is:
proverb
'saying' is the definition.
(I know that proverb is a type of saying)
'pet dog wears lead' is the wordplay.
'pet dog' becomes 'rover' (Rover is a common name for a pet dog. I am not sure about the 'pet' bit.).
'wears' indicates putting letters inside.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' placed inside 'pb' is 'PROVERB'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for proverb that I've seen before include "Old saw" , "Wise saying" , "Short saying embodying a supposed truth" , "Adage, pithy saying" , "Traditional saying" .)