Tough one to crack, perhaps (4)
I believe the answer is:
safe
This clue might be a double definition.
'tough one' is the first definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'crack perhaps' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'to' is the link.
Can you help me to learn more?
(Other definitions for safe that I've seen before include "Peter" , "Free from harm" , "Metal box for storing valuables in" , "Secure - place to keep cash" , "Strongbox; secure" .)