Force failing to open lock (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'force failing to open' is the wordplay.
'force' becomes 'stress' (stress is a kind of force).
'failing to open' means to remove the first letter (the opening letter is removed).
'stress' with its initial letter removed is 'TRESS'.
(Other definitions for tress that I've seen before include "Bit of hair" , "Curl - lock" , "Braid of hair" , "Rests (anag.)" , "Obsolete support" .)