Roundhead, nominally, with battle souvenir (5)
I believe the answer is:
oscar
'roundhead' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'nominally with battle souvenir' is the wordplay.
'nominally' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'battle souvenir' becomes 'scar' (I've seen this in another clue).
'o'+'scar'='OSCAR'
Can you help me to learn more?
(Other definitions for oscar that I've seen before include "Film industry award" , "- - Peterson, pianist" , "Develop again" , "Comms code for "O"" , "Man" .)