One displaces equestrian in tournament, taking the lead (6)
I believe the answer is:
ouster
'one displaces equestrian' is the definition.
I can't tell whether this definition defines the answer.
'tournament taking the lead' is the wordplay.
'tournament' becomes 'ous' (I can't explain this - if you can you should believe this answer much more).
'taking' is a deletion indicator (take can mean remove or delete).
'lead' becomes 'tether' (lead can mean a tether or leash).
'tether' with 'the' removed is 'ter'.
'ous'+'ter'='OUSTER'
'in' acts as a link.
Can you help me to learn more?
(Another definition for ouster that I've seen is " Ejection".)