Secure former Earl whose horse has bolted (6)
I believe the answer is:
batten
'secure' is the definition.
(I know that secure can be written as batten)
I can't explain the remainder of the clue.
(Other definitions for batten that I've seen before include "Piece of timber" , "Wood strip; fasten for advantage (on)" , "Wooden fastening strip" , "Wooden stiffening strip" , "Close securely, say the hatches" .)