Hibernian flag planted on peak of Himalayas (5)
I believe the answer is:
irish
'hibernian' is the definition.
(I've seen this in another clue)
'flag planted on peak of himalayas' is the wordplay.
'flag' becomes 'iris' (type of iris flower).
'planted on' says to put letters next to each other.
'peak of' suggests taking the first letters (top in a down clue).
The first letter of 'himalayas' is 'h'.
'iris'+'h'='IRISH'
(Other definitions for irish that I've seen before include "Like Wilde or Yeats" , "From Dublin, perhaps" , "From Cork, for instance" , "Native to Erin" , "Relating to Ireland" .)