Shrub unknown in area beside a river (6)
I believe the answer is:
azalea
'shrub' is the definition.
(azalea is a kind of shrub)
'beside a river' is the wordplay.
'beside' becomes 'az' (I can't explain this - if you can you should believe this answer much more).
'river' becomes 'Lea' (River Lea in South East England).
'az'+'a'+'lea'='AZALEA'
'unknown in area' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for azalea that I've seen before include "Bright-flowered shrub" , "Ericaceous plant" , "Rhododendron-type shrub" , "Rhododendron-like shrub" , "Ornamental lime-hating shrub" .)