Saw rambler in pub, disheartened (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'rambler in pub disheartened' is the wordplay.
'rambler' becomes 'rover' (I've seen this before).
'in' is an insertion indicator.
'disheartened' suggests removing the centre.
'pub' with its centre taken out is 'pb'.
'rover' put into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "Short saying embodying a supposed truth" , "Wise saying" , "Traditional saying" , "Maxim" , "Short saying expressing a supposed truth" .)