Lock theatre's spare boxes (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'theatre's spare boxes' is the wordplay.
'boxes' indicates the answer is hidden within the clue (to box can mean to contain).
'TRESS' is hidden in the letters of 'theatres spare'.
(Other definitions for tress that I've seen before include "Switch" , "Long lock or plait of hair" , "Obsolete support" , "Rests (anag.)" , "Curl - lock" .)