Energy finding an outlet in sport (8)
I believe the answer is:
eventing
'sport' is the definition.
(I've seen this before)
'energy finding an outlet in' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcn).
'finding an' becomes 'ing' (I can't explain this - if you can you should believe this answer much more).
'outlet' becomes 'vent' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'e'+'ing'='eing'
'eing' enclosing 'vent' is 'EVENTING'.
Can you help me to learn more?
(Other definitions for eventing that I've seen before include "A three-phase equestrian sport" , "Sport consisting of dressage, cross country and show jumping" , "Equestrian competition" .)