Hothead stands by flag from the Emerald Isle (5)
I believe the answer is:
irish
'from the emerald isle' is the definition.
(I've seen this before)
'hothead stands by flag' is the wordplay.
'stands' indicates taking the first letters (I've seen 'standing' mean this).
'by' means one lot of letters go next to another.
'flag' becomes 'iris' (type of iris flower).
The first letter of 'hothead' is 'h'.
'h' put after 'iris' is 'IRISH'.
(Other definitions for irish that I've seen before include "Hi, Sir, ar you a Celt?" , "Joyce was" , "Celtic language" , "One setter" , "Kind of whiskey" .)