Good wet weather for cereal crop (5)
I believe the answer is:
grain
'for cereal crop' is the definition.
(I've seen this before)
'good wet weather' is the wordplay.
'good' becomes 'g' (abbreviation).
'wet weather' becomes 'rain' (rain is wet weather).
'g'+'rain'='GRAIN'
(Other definitions for grain that I've seen before include "Cereal as food" , "Small particle - weight" , "Eg, wheat" , "Small hard particle, of rice perhaps" , "Cereals" .)