Mistakes made by English kings at start of actions (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'english kings at start of actions' is the wordplay.
'english kings' becomes 'err' (I can't justify this - if you can you should give a lot more credence to this answer).
'start of' indicates taking the first letters.
The first letter of 'actions' is 'a'.
'err'+'at'+'a'='ERRATA'
'made by' acts as a link.
(Other definitions for errata that I've seen before include "List of misprints" , "(Printing) errors" , "Mistakes (Latin)" , "List of errors" , "mistakes in print" .)