In this part of the UK everyone is an Irishman (5)
I believe the answer is:
niall
'irishman' is the definition.
(common Irish name)
'in this part of the uk everyone is an' is the wordplay.
'in' indicates putting letters inside.
'this part of the uk' becomes 'n' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'everyone' becomes 'all' (all people).
'is an' becomes 'i' (Roman numeral for one).
'n'+'all'='nall'
'nall' placed around 'i' is 'NIALL'.
Can you help me to learn more?
(Another definition for niall that I've seen is " Irishman".)