Plant oddly really prompt to come up (6)
I believe the answer is:
poplar
'plant' is the definition.
(poplar matches up with the definition plant)
'oddly really prompt to come up' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'to come up' says the letters should be written in reverse.
'really'+'prompt'='reallyprompt'
The alternate letters of 'reallyprompt' are 'ralpop'.
'ralpop' back-to-front is 'POPLAR'.
(Other definitions for poplar that I've seen before include "Tree as tulip-tree, aspen" , "Tree with light, soft wood and catkins" , "Tree - London district" , "tree found mostly in the northern hemisphere" , "Softwood tree with catkins" .)