State emphatically split during a glut (10)
I believe the answer is:
asseverate
'state emphatically' is the definition.
(to asseverate is to declare something)
'split during a glut' is the wordplay.
'split' becomes 'sever' (sever can mean to split or separate).
'during' is an insertion indicator.
'glut' becomes 'sate' (I've seen this in another clue).
'a'+'sate'='asate'
'sever' put within 'asate' is 'ASSEVERATE'.
(Other definitions for asseverate that I've seen before include "Solemnly say" , "Claim" , "State categorically" , "Swear" , "Solemnly declare" .)