Bush's ideal running mate originally (8)
I believe the answer is:
buddleia
'bush's' is the definition.
(buddleia is a kind of bush)
'ideal running mate originally' is the wordplay.
I cannot quite see how this works, but
an anagram of 'ideal' is 'dleia' which is within the answer.
'mate' could be 'bud' (short for buddy) and 'bud' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for buddleia that I've seen before include "Shrub with clusters of fragrant lilac, white or yellow flowers" , "flowering plant" , "Flowering shrub" , "Butterfly bush" , "Small coin of Mexico, Brazil" .)