Saw dog in empty pub (7)
I believe the answer is:
proverb
'saw' is the definition.
(a saw can mean a proverb or maxim)
'dog in empty pub' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'in' means one lot of letters goes inside another.
'empty' suggests removing the centre.
'pub' with its centre removed is 'pb'.
'rover' placed inside 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "OT saying" , "Wise saying" , "Saying encapsulating a general truth" , "Short saying embodying a supposed truth" , "Short saying expressing a supposed truth" .)