Who might participate in trial with entering running relay (6)
I believe the answer is:
lawyer
'who might participate in trial' is the definition.
(a lawyer is involved in a trial)
'with entering running relay' is the wordplay.
'with' becomes 'w' (abbreviation).
'entering' means one lot of letters goes inside another.
'running' is an anagram indicator.
'relay' with letters rearranged gives 'layer'.
'w' put within 'layer' is 'LAWYER'.
(Other definitions for lawyer that I've seen before include "Solicotor or barrister" , "Legal professional" , "One provides legal advice" , "Man at the bar?" , "barman" .)