To summarise, a military policeman’s no dunderhead! (5)
I believe the answer is:
recap
'to summarise' is the definition.
(recapping is a kind of summarizing)
'a military policeman's no dunderhead' is the wordplay.
'a military policeman' becomes 'redcap' (redcap is a kind of military policeman).
'no' indicates named letters should be taken away.
'dunderhead' becomes 'd' (head letter of 'dunder').
'redcap' with 'd' taken away is 'RECAP'.
(Other definitions for recap that I've seen before include "Go over again with backward pacer" , "go over chief points again" , "Go over the main points again, in short" , "Provide summary" , "Go over study material again" .)