Get on a horse held by me (6)
I believe the answer is:
manage
'get on' is the definition.
(I've seen this before)
'a horse held by me' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'held by' indicates putting letters inside.
'a'+'nag'='anag'
'anag' put inside 'me' is 'MANAGE'.
(Other definitions for manage that I've seen before include "Run; cope" , "Be in overall control of" , "Organise" , "Control, direct, run" , "Handle, oversee" .)