Plant unknown in Northern Ireland area (6)
I believe the answer is:
zinnia
'plant' is the definition.
(zinnia is a kind of plant)
'unknown in northern ireland area' is the wordplay.
'unknown' becomes 'z' (algebra).
'northern ireland' becomes 'NI' (abbreviation).
'area' becomes 'a' (maths abbreviation).
'z'+'in'+'ni'+'a'='ZINNIA'
(Other definitions for zinnia that I've seen before include "Showy flower (last in any index?)" , "showy garden feature" , "Tropical plant" , "bloomers" , "Flowering plant" .)