Old enclosures originally employed identical means of securing access (4,6)
I believe the answer is:
open sesame
'access' is the definition.
(thesaurus)
'old enclosures originally employed identical' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'enclosures' becomes 'pens' (pen is a kind of enclosure).
'originally' says to take the initial letters.
'identical' becomes 'same' (similar in meaning).
The first letter of 'employed' is 'e'.
'o'+'pens'+'e'+'same'='OPEN SESAME'
'means of securing' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for open sesame that I've seen before include "Magical words" , "Spell unlocking doors" , "Magic password" , "Spell from Ali Baba and the Forty Thieves" , "Words of Ali Baba" .)