Appeal for the benefit of a dunderhead in prison (5)
I believe the answer is:
cadge
'appeal for the benefit' is the definition.
I can't judge whether this definition defines the answer.
'a dunderhead in prison' is the wordplay.
'a dunderhead' becomes 'd' (head letter of 'dunder').
'in' indicates putting letters inside.
'prison' becomes 'cage' (cage is a kind of prison).
'd' placed into 'cage' is 'CADGE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for cadge that I've seen before include "Sponge (money, etc)" , "Get money by sponging" , "Bum" , "Scrounge, sponge" , "get free" .)