Busts are possibly hard to understand (8)
I believe the answer is:
abstruse
'hard to understand' is the definition.
(I've seen this before)
'busts are possibly' is the wordplay.
'possibly' indicates an anagram.
'busts'+'are'='bustsare'
'bustsare' anagrammed gives 'ABSTRUSE'.
(Other definitions for abstruse that I've seen before include "Hard to understand, recondite" , "Difficult to penetrate or understand" , "Hard to understand, complicated" , "Arcane" , "Hard to understand, perhaps profound" .)