In this event, one may be for it (4,4)
I believe the answer is:
high jump
'in this event one may be for it' is the definition.
'high jump' can be an answer for 'event' (high jump is a kind of event). I'm not certain of the remainder of the definition.
'in this event' is the wordplay.
'in this' becomes 'high' (I can't explain this - if you can you should believe this answer much more).
'event' becomes 'jump' (jump is a kind of event).
'high'+'jump'='HIGH JUMP'
Can you help me to learn more?
(Other definitions for high jump that I've seen before include "Event for the upwardly mobile" , "Athletics event (for those being punished?)" , "be in trouble for this?" , "A severe reprimand" , "bar activity?" .)