Half of crackers fail to go off? Ridiculous! (8)
I believe the answer is:
farcical
'ridiculous' is the definition.
('farcical' can be a synonym of 'ridiculous')
'half of crackers fail to go off?' is the wordplay.
'half of' indicates taking half.
'to go off?' indicates an anagram (I've seen 'go off' mean this).
'crackers' with half the letters taken is 'crac'.
'crac'+'fail'='cracfail'
'cracfail' is an anagram of 'FARCICAL'.
(Other definitions for farcical that I've seen before include "Utterly ridiculous like bad comedy" , "4 [COMIC]" , "Ridiculous as in broad comedy" , "Racial FC is ridiculous" , "Quite ridiculous, ludicrous" .)