He's not crooked - just the opposite (6)
I believe the answer is:
honest
'just the opposite' is the definition.
I can't judge whether this defines the answer.
'he's not crooked' is the wordplay.
'crooked' indicates anagramming the letters (a crooked or disrupted version of the letters).
'hes'+'not'='hesnot'
'hesnot' is an anagram of 'HONEST'.
Can you help me to learn more?
(Other definitions for honest that I've seen before include "Not dissembling" , "Truthful in manner" , "Genuine and trustworthy" , "Impartial intermediary" , "really!" .)