Swear idiot always spilled tea (10)
I believe the answer is:
asseverate
'swear' is the definition.
(I've seen this before)
'idiot always spilled tea' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'always' becomes 'ever' (synonyms).
'spilled' indicates anagramming the letters.
'tea' with letters rearranged gives 'ate'.
'ass'+'ever'+'ate'='ASSEVERATE'
(Other definitions for asseverate that I've seen before include "State categorically" , "Claim" , "Solemnly declare" , "Solemnly say" , "Swear" .)