A sudden outbreak broke king in (5)
I believe the answer is:
burst
'a sudden outbreak' is the definition.
(I've seen this in another clue)
'broke king in' is the wordplay.
'broke' becomes 'bust' (synonyms).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'in' means one lot of letters goes inside another.
'bust' enclosing 'r' is 'BURST'.
(Other definitions for burst that I've seen before include "Explode (as 1 acrosses do)" , "Split asunder" , "Puncture, explode" , "Blow up" , "Fit" .)