Seeing King flee, make certain to follow (5)
I believe the answer is:
ensue
'to follow' is the definition.
(ensue can mean to follow or happen as a result)
'seeing king flee make certain' is the wordplay.
'seeing king' becomes 'R' (abbreviation of Latin rex).
'flee' is a deletion indicator (some letters flee or run away).
'make certain' becomes 'ensure' (synonyms).
'ensure' with 'r' taken out is 'ENSUE'.
(Other definitions for ensue that I've seen before include "Happen afterwards as a result" , "Follow subsequently" , "Follow afterwards" , "Happen as consequence or afterward" , "Follow as a consequence" .)