Saw dog in pub — not acceptable (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'dog in pub not acceptable' is the wordplay.
'dog' becomes 'rover' (I've seen this before**).
'in' means one lot of letters goes inside another.
'not' suggests deleting specific letters.
'acceptable' becomes 'u' ((Film classification)).
'pub' with 'u' taken away is 'pb'.
'rover' going inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Familiar saying" , "Aphorism" , "Pithy saying expressing a general truth" , "encapsulate wisdom" , "'Adage, saw (7)'" .)