Very stuck in one crack (5)
I believe the answer is:
solve
'crack' is the definition.
(crack can mean to solve or decipher)
'very stuck in one' is the wordplay.
'very' becomes 'v' (common abbreviation).
'stuck in' means one lot of letters goes inside another.
'one' becomes 'sole' (synonyms).
'v' inserted into 'sole' is 'SOLVE'.
(Other definitions for solve that I've seen before include "Loves (anag) - to find an answer" , "Unravel (a problem)" , "Settle" , "Get answer to (a clue)" , "Figure out the answer" .)