Willpower about to crack (7)
I believe the answer is:
resolve
'willpower' is the definition.
(resolve can mean willpower or determination)
'about to crack' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to crack' becomes 'solve' (crack can mean to solve or decipher).
're'+'solve'='RESOLVE'
(Other definitions for resolve that I've seen before include "find an answer" , "Fix" , "Firmness of purpose" , "Make a firm decision" , "Find the solution (to)" .)