On trial, a criminal must be reasonable (8)
I believe the answer is:
rational
'reasonable' is the definition.
(associated in meaning)
'on trial a criminal' is the wordplay.
'criminal' indicates an anagram.
'on'+'trial'+'a'='ontriala'
'ontriala' anagrammed gives 'RATIONAL'.
'must be' acts as a link.
(Other definitions for rational that I've seen before include "17 [INTELLIGENT]" , "Using reason" , "Sane and logical" , "Requiring the use of the mind" , "Endowed with reason" .)