Saw dog biting lead (7)
I believe the answer is:
proverb
'saw' is the definition.
(synonyms)
'dog biting lead' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'biting' means one lot of letters goes inside another (bite can mean to puncture or penetrate).
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rover' inserted into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "'Adage, saw (7)'" , "OT saying" , "Generally true saying" , "Wise saying" , "Saying of great truth" .)