Force lock after first sign of sticking (6)
I believe the answer is:
stress
'force' is the definition.
(I know that stress is a type of force)
'lock after first sign of sticking' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'after' means one lot of letters go next to another.
'first sign of' indicates taking the first letters.
The initial letter of 'sticking' is 's'.
'tress' after 's' is 'STRESS'.
(Other definitions for stress that I've seen before include "Dwell on" , "Physical or mental over-exertion" , "Strain or accentuate" , "Cause of crack" , "State mental or emotional strain" .)