Managed to include old horse (4)
I believe the answer is:
roan
'horse' is the definition.
(I know that roan is a type of horse)
'managed to include old' is the wordplay.
'managed' becomes 'ran' (both can mean organised or was in charge of).
'to include' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ran' enclosing 'o' is 'ROAN'.
(Other definitions for roan that I've seen before include "Nora (anag) - horse" , "Horse with reddish-brown coat" , "Horse with main colour interspersed with hairs of another" , "Horse with coat speckled with white or grey" , "(Horse) white-sprinkled coat" .)