Transmission expert left to reflect during trial (8)
I believe the answer is:
telecast
'transmission' is the definition.
(a telecast is a TV transmission)
'expert left to reflect during trial' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'left' becomes 'l' (common abbreviation).
'to reflect' says the letters should be written backwards.
'during' indicates putting letters inside.
'trial' becomes 'test' (synonyms).
'ace'+'l'='acel'
'acel' in reverse letter order is 'leca'.
'leca' put inside 'test' is 'TELECAST'.
(Other definitions for telecast that I've seen before include "TV transmission" , "TV programme?" , "TV show" , "TV broadcast" .)