Setter picked up vivid plant (9)
I believe the answer is:
eyebright
'plant' is the definition.
'setter picked up vivid' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'picked up' indicates a 'sounds like' (homophone) clue.
'vivid' becomes 'bright' (synonyms).
'i' is a homophone of 'eye'.
'eye'+'bright'='EYEBRIGHT'
(Other definitions for eyebright that I've seen before include "Small plant" , "that could help the viewer" , "White-flowered plant" , "A plant; they're big (anag.)" , "Wild flower - get by hire (anag)" .)