Trial defendant finally fixed up (4)
I believe the answer is:
test
'trial' is the definition.
(I know that trial can be written as test)
'defendant finally fixed up' is the wordplay.
'finally' indicates one should take the final letters.
'fixed' becomes 'set' (both can mean decided or determined).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The last letter of 'defendant' is 't'.
't'+'set'='tset'
'tset' reversed gives 'TEST'.
(Other definitions for test that I've seen before include "Examine to play international cricket match" , "water flowing through Hampshire" , "big game" , "type of examination" , "Trial - international game" .)