Plants one's managed to gather, so heading north (8)
I believe the answer is:
narcissi
'plants' is the definition.
The answer is an example.
'one's managed to gather so heading north' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'managed' becomes 'ran' (I've seen this before**).
'to gather' means one lot of letters goes inside another.
'so' becomes 'sic' ('sic' can be a synonym of 'so').
'heading north' says the letters should be written in reverse.
'is'+'ran'='isran'
'isran' going around 'sic' is 'issicran'.
'issicran' reversed gives 'NARCISSI'.
(Other definitions for narcissi that I've seen before include "Captain in Moby Dick" , "Scan iris for the spring flowers" , "Flowers including daffodils and jonquils" , "Spring-flowering plants" , "Flowers named after Greek youth in love with his own appearance" .)