Experiment with running over most of the competition (9)
I believe the answer is:
triathlon
'competition' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'experiment with running over most of the' is the wordplay.
'experiment' becomes 'trial' (trial is a kind of experiment).
'with running' becomes 'on' (eg when a machine is running it is on. I am not sure about the 'with' bit.).
'over' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'trial'+'on'='trialon'
'the' with its last letter taken away is 'th'.
'trialon' enclosing 'th' is 'TRIATHLON'.
Can you help me to learn more?
(Other definitions for triathlon that I've seen before include "Athletics event" , "Multi-disciplined Olympic event since 2000" , "Sporting event" , "Sporting contest comprising swimming, cycling and running" , "Triple athletic event" .)