Everybody featured in trial least likely to be believable (7)
I believe the answer is:
tallest
'least likely to be believable' is the definition.
I can't judge whether this definition defines the answer.
'everybody featured in trial' is the wordplay.
'everybody' becomes 'all' (all people).
'featured in' is an insertion indicator.
'trial' becomes 'test' (synonyms).
'all' inserted into 'test' is 'TALLEST'.
Can you help me to learn more?
(Other definitions for tallest that I've seen before include "most unlikely" , "Top out" , "Sell tat (anag.)" , "Of extreme height" , "Of greatest vertical extent" .)