Biggest trial including everyone (7)
I believe the answer is:
tallest
'biggest' is the definition.
Both the definition and answer are superlative adjectives. Perhaps you can see an association between them that I can't see?
'trial including everyone' is the wordplay.
'trial' becomes 'test' (synonyms).
'including' indicates putting letters inside.
'everyone' becomes 'all' (all people).
'test' enclosing 'all' is 'TALLEST'.
Can you help me to learn more?
(Other definitions for tallest that I've seen before include "Top out" , "Of greatest height" , "Of extreme height" , "Sell tat (anag.)" , "most unlikely" .)