Does crack? Bad luck, son (7)
I believe the answer is:
unlocks
'does crack?' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Perhaps you can see an association between them that I don't see?
'bad luck son' is the wordplay.
'bad' indicates anagramming the letters.
'luck'+'son'='luckson'
'luckson' anagrammed gives 'UNLOCKS'.
Can you help me to learn more?
(Other definitions for unlocks that I've seen before include "Unfastens" , "coil of hair, not black hair" , "Releases with a key" .)