Terrible slab may break (7)
I believe the answer is:
abysmal
'terrible' is the definition.
(abysmal means very bad)
'slab may break' is the wordplay.
'break' is an anagram indicator.
'slab'+'may'='slabmay'
'slabmay' anagrammed gives 'ABYSMAL'.
(Other definitions for abysmal that I've seen before include "Very bad or bottomless" , "Extreme and deplorable as ignorance may be" , "Dreadful" , "Shockingly bad" , "Pathetic" .)