Biggest gear breaking up when on trial (8)
I believe the answer is:
greatest
'biggest' is the definition.
('great' can be a synonym of 'big')
'gear breaking up when on trial' is the wordplay.
'breaking up' indicates an anagram.
'when on' means one lot of letters go next to another.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'gear' anagrammed gives 'grea'.
'grea'+'test'='GREATEST'
(Other definitions for greatest that I've seen before include "Get stare from the mightiest" , "Most important!" , "Not to be surpassed" , "''But the . . . . . . . . of these is love'' - Bible" , "Largest or most distinguished" .)