Northern Ireland set originally labelling a flowering plant (7)
I believe the answer is:
nigella
'flowering plant' is the definition.
(I know that nigella is a type of flowering plant)
'northern ireland set originally labelling a' is the wordplay.
'northern ireland' becomes 'NI' (abbreviation).
'set' becomes 'gel' (both can mean to harden).
'originally' suggests taking the first letters.
The first letter of 'labelling' is 'l'.
'ni'+'gel'+'l'+'a'='NIGELLA'
(Other definitions for nigella that I've seen before include "TV personality" , "Woman" , "Flower and TV chef" , "Flower found in legal" , "plant" .)