Worry safecracker's opening lock (6)
I believe the answer is:
stress
'worry' is the definition.
(I've seen this before)
'safecracker's opening lock' is the wordplay.
'opening' suggests taking the first letters.
'lock' becomes 'tress' (both can mean a piece of hair).
The initial letter of 'safecrackers' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "result of being overworked" , "Strain or accentuate" , "Feeling of pressure" , "'Worry, pressure (6)'" , "nervous tension" .)