Hibernian flag on hospital (5)
I believe the answer is:
irish
'hibernian' is the definition.
(I've seen this in another clue)
'flag on hospital' is the wordplay.
'flag' becomes 'iris' (type of iris flower).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'hospital' becomes 'H' (abbreviation for hospital).
'iris'+'h'='IRISH'
(Other definitions for irish that I've seen before include "One setter" , "Reddish-brown dog" , "Native to Erin" , "Eg, native to Dublin" , "Relating to Ireland" .)