Dog restrained by lead beginning to sniff book (8)
I believe the answer is:
proverbs
'book' is the definition.
(I know that Proverbs is a book)
'dog restrained by lead beginning to sniff' is the wordplay.
'dog' becomes 'rover' (Rover is a common name for a pet dog).
'restrained by' is an insertion indicator.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'beginning to' says to take the initial letters.
The initial letter of 'sniff' is 's'.
'rover' inserted into 'pb' is 'proverb'.
'proverb'+'s'='PROVERBS'
(Other definitions for proverbs that I've seen before include "Gnomes" , "Old Testament book ascribed to Solomon, or well-known sayings" , "Adages, saws" , "Truthful expressions" , "More than one saw" .)