Crack or unravel (5)
I believe the answer is:
solve
I believe this is a double definition.
'crack' is the first definition.
(crack can mean to solve or decipher)
'unravel' is the second definition.
(I've seen this in another clue)
(Other definitions for solve that I've seen before include "Unravel (a problem)" , "Find the answer to a mystery" , "Loves (anag) - to find an answer" , "Complete crossword" , "Deal with (a problem)" .)