Like this clue, a trial? (6)
I believe the answer is:
across
'like this clue' is the definition.
(I've seen this in another clue)
'a trial?' is the wordplay.
'trial?' becomes 'cross' (synonyms).
'a'+'cross'='ACROSS'
(Other definitions for across that I've seen before include "Clues like this" , "sort of clue?" , "Horizontally" , "...this is 'Number Ten'" , "Transverse" .)