Force opening of security lock (6)
I believe the answer is:
stress
'force' is the definition.
(I know that stress is a type of force)
'opening of security lock' is the wordplay.
'opening of' says to take the initial letters.
'lock' becomes 'tress' (both can mean a piece of hair).
The first letter of 'security' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Cause of crack" , "result of being overworked" , "Particular emphasis" , "Emotional tension" , "Emphasis on anxiety?" .)