Saw a dunderhead get on (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'a dunderhead get on' is the wordplay.
'dunderhead' becomes 'd' (head letter of 'dunder').
'get on' becomes 'age' ('age' can be a synonym of 'get on'**).
'a'+'d'+'age'='ADAGE'
(Other definitions for adage that I've seen before include "Saw, proverb" , "Maxim, aphorism" , "Proverb, maxim" , "say it again and again!" , "Slogan" .)