A truly incomplete state (4)
I believe the answer is:
aver
'state' is the definition.
(thesaurus)
'a truly incomplete' is the wordplay.
'truly' becomes 'very' (I've seen this before).
'incomplete' means to remove the last letter.
'very' with its last letter removed is 'ver'.
'a'+'ver'='AVER'
(Other definitions for aver that I've seen before include "State (to be the case)" , "State firmly - don't rave" , "say, with conviction" , "Maintain - state" , "Formally assert" .)