Saw rambler in empty bar (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'rambler in empty bar' is the wordplay.
'rambler' becomes 'rover' (I've seen this before).
'in' is an insertion indicator.
'empty' says to hollow out the word (remove centre letters).
'bar' becomes 'pub' (I've seen this before).
'pub' with its centre taken out is 'pb'.
'rover' placed into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Short saying embodying a supposed truth" , "gnome" , "Saying of great truth" , "Pithy saying expressing a general truth" , "Maxim" .)