Force small lock (6)
I believe the answer is:
stress
'force' is the definition.
(I know that stress is a type of force)
'small lock' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lock' becomes 'tress' (both can mean a piece of hair).
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Draw attention to" , "Particular emphasis" , "Underline - tension" , "Emphasis on anxiety?" , "Emphasize" .)