Down under, the Briton may not take my part (5)
'down under' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'the briton may not take my part' is the wordplay.
'the briton' becomes 'pom' (pom is a kind of briton).
'may not' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'take' indicates putting letters inside.
'part' indicates taking the first letters.
The first letter of 'my' is 'm'.
'pomy' enclosing 'm' is 'POMMY'.
Can you help me to learn more?
(Other definitions for pommy that I've seen before include "British person (Australian colloquial)", "Abusive term for English", "Victorian Englishman".)