Nationality of girl at Holyhead (5)
I believe the answer is:
irish
'nationality' is the definition.
(Irish is an example)
'girl at holyhead' is the wordplay.
'girl' becomes 'iris' (girl's name).
'at' means one lot of letters go next to another.
'holyhead' becomes 'h' (head letter of 'holy').
'iris'+'h'='IRISH'
'of' acts as a link.
(Other definitions for irish that I've seen before include "Eg, native to Dublin" , "said to be lucky" , "Gaelic language" , "Nationality" , "Like Wilde or Yeats" .)