What could protect trainer round very wild horse with energy (8)
I believe the answer is:
overshoe
'what could protect trainer' is the definition.
(an overshoe might be worn over a trainer)
'round very wild horse with energy' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'very' becomes 'v' (abbreviation).
'wild' indicates an anagram.
'with' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'horse' anagrammed gives 'rshoe'.
'rshoe' enclosing 'e' is 'ershoe'.
'o'+'v'+'ershoe'='OVERSHOE'
(Other definitions for overshoe that I've seen before include "Galosh" , "one covering Oxford?" , "Waterproof footwear" , "Re hooves? (anag)" , "which may be outside Oxford" .)