Force special lock (6)
I believe the answer is:
stress
'force' is the definition.
(stress is a kind of force)
'special lock' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'lock' becomes 'tress' (both can mean a piece of hair).
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "'Worry, pressure (6)'" , "Emotional tension" , "Tension - accent" , "Strain or accentuate" , "Emphasis: result often of being under pressure" .)