Declare to be cut off, in a wrong seat (10)
I believe the answer is:
'declare' is the definition.
(I've seen this before)
'cut off in a wrong seat' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'in' is an insertion indicator.
'wrong' indicates an anagram.
'seat' anagrammed gives '[seat]'.
'sever' going within 'asate' is 'ASSEVERATE'.
'to be' is the link.
(Other definitions for asseverate that I've seen before include "State categorically" , "Solemnly declare" , "Swear" , "Solemnly say" , "Claim" .)