Difficult, but ought to be cracked (5)
I believe the answer is:
tough
'difficult' is the definition.
(similar in meaning)
'ought to be cracked' is the wordplay.
'to be cracked' indicates an anagram.
'ought' is an anagram of 'TOUGH'.
'but' acts as a link.
(Other definitions for tough that I've seen before include "Hard to cut or chew" , "Hard cheese" , "Rugged" , "Strong - ruffian" , "Strong or leathery" .)