Force unopened lock (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'force unopened' is the wordplay.
'force' becomes 'stress' (stress is a kind of force).
'unopened' means to remove the first letter (without opening letter).
'stress' with its initial letter removed is 'TRESS'.
(Other definitions for tress that I've seen before include "is a bit of a shock" , "Obsolete support" , "Long piece of hair" , "Bit of hair" , "Braid of hair" .)