Changed alone, with red-back in a shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(I've seen this before)
'changed alone with red-back' is the wordplay.
'changed' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'back' says the letters should be written backwards.
'red' in reverse letter order is 'der'.
'alone' with letters rearranged gives 'olean'.
'olean'+'der'='OLEANDER'
'in a' is the link.
(Other definitions for oleander that I've seen before include "poisonous plant" , "Flowering shrub from olden era, oddly" , "No leader, oddly, for the flowering shrub" , "bloomer" , "No dealer in evergreen shrub" .)