"Then what happened!?" (3)
I believe the answer is:
and
'then what happened?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for and that I've seen before include "Word to connect" , "Joining word" , "it needs wholly positive input for positive result" , "A very common conjunction" , "It connects words, clauses or sentences" .)