Clear-headed on trial, a criminal (8)
I believe the answer is:
rational
'clear-headed' is the definition.
(I've seen this before)
'on trial a criminal' is the wordplay.
'criminal' indicates anagramming the letters.
'on'+'trial'+'a'='ontriala'
'ontriala' is an anagram of 'RATIONAL'.
(Other definitions for rational that I've seen before include "Reasonable - sane" , "Sensible, based on reason" , "Capable of thought" , "Logical; sensible" , "Endowed with reason" .)