Ultimately, couldn't care less about trial (4)
I believe the answer is:
test
'trial' is the definition.
('test' can be a synonym of 'trial')
'ultimately couldn't care less about' is the wordplay.
'ultimately' says to take the final letters.
The last letters of 'couldnt care less about' are 'TEST'.
(Other definitions for test that I've seen before include "Check" , "Trial; exam" , "Assess - examination" , "Cricket match; exam" , "big game" .)