A good hail does for plant (5)
I believe the answer is:
agave
'plant' is the definition.
The answer is an example.
'a good hail' is the wordplay.
'good' becomes 'g' (abbreviation).
'hail' becomes 'ave' (greeting in Latin).
'a'+'g'+'ave'='AGAVE'
'does for' acts as a link.
(Other definitions for agave that I've seen before include "Tropical plant" , "plant in Mexico" , "A cactus" , "Succulent; daughter of Cadmus" , "Spiky succulent" .)