A cereal crop, except on pastureland (6)
I believe the answer is:
barley
'a cereal crop' is the definition.
(I've seen this before)
'except on pastureland' is the wordplay.
'except' becomes 'bar'.
'on' means one lot of letters go next to another.
'pastureland' becomes 'ley' (synonyms).
'bar'+'ley'='BARLEY'
(Other definitions for barley that I've seen before include "The grain may make you bleary" , "Cereal crop" , "A hardy grass" , "Cereal used in brewing" , "Grain used in beer-making" .)