A rift so restricting a northern plant (10)
I believe the answer is:
agapanthus
'plant' is the definition.
(agapanthus is a plant genus)
'a rift so restricting a northern' is the wordplay.
'rift' becomes 'gap' (I've seen this in another clue).
'so' becomes 'thus' (synonyms).
'restricting' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'gap'+'thus'='gapthus'
'a'+'n'='an'
'gapthus' placed around 'an' is 'gapanthus'.
'a'+'gapanthus'='AGAPANTHUS'
(Other definitions for agapanthus that I've seen before include "Southern African plant of the lily family -- haunts a gap (anag)" , "Flowers" , "garden plant" , "Bloomer" , "flower" .)