Origin surrounding the first horse (5)
I believe the answer is:
steed
'horse' is the definition.
(I know that steed is a type of warhorse)
'origin surrounding the first' is the wordplay.
'origin' becomes 'seed' (seed is a kind of origin).
'surrounding' is an insertion indicator.
'first' says to take the initial letters.
The first letter of 'the' is 't'.
'seed' enclosing 't' is 'STEED'.
(Other definitions for steed that I've seen before include "A mount (poet.)" , "Stallion" , "Horse, mount" , "Literary term for horse" , "Warhorse" .)