Lock as part of theatre's security (5)
I believe the answer is:
tress
'lock' is the definition.
(I've seen this before)
'part of theatre's security' is the wordplay.
'part of' indicates a hidden word.
'TRESS' can be found hidden inside 'theatres security'.
'as' is the link.
(Other definitions for tress that I've seen before include "Rests (anag.)" , "is a bit of a shock" , "Long piece of hair" , "Curl - lock" , "Long lock or plait of hair" .)