Stupid person the day before helping to produce solemn statement (12)
I believe the answer is:
asseveration
'solemn statement' is the definition.
(I've seen this before)
'stupid person the day before helping' is the wordplay.
'stupid person' becomes 'ass' (I've seen this before).
'the day before' becomes 'eve' (I've seen this before).
'helping' becomes 'ration' (I've seen this before).
'ass'+'eve'+'ration'='ASSEVERATION'
'to produce' is the link.
(Other definitions for asseveration that I've seen before include "Solemn statement" , "Pronouncement" .)