Occasional crop said to be unreliable (8)
I believe the answer is:
sporadic
'occasional' is the definition.
(thesaurus)
'crop said to be unreliable' is the wordplay.
'to be unreliable' is an anagram indicator.
'crop'+'said'='cropsaid'
'cropsaid' with letters rearranged gives 'SPORADIC'.
(Other definitions for sporadic that I've seen before include "infrequent" , "Having separate patches" , "Isolated; intermittent" , "Occurring here and there, scattered" , "Picadors being scattered, here and there" .)