Plant disease bad at first, not serious after (6)
I believe the answer is:
blight
'plant disease' is the definition.
(blight is a kind of plant disease)
'bad at first not serious after' is the wordplay.
'at first' indicates taking the first letters.
'not serious' becomes 'light' ('light' is the opposite to 'serious').
'after' means one lot of letters go next to another.
The first letter of 'bad' is 'b'.
'b'+'light'='BLIGHT'
(Other definitions for blight that I've seen before include "Curse - plague" , "Spoil - plant disease" , "Withering disease (in plants)" , "Disease - pest" , "Affliction" .)