Unlocks old cages (5)
I believe the answer is:
opens
'unlocks' is the definition.
(I've seen this before)
'old cages' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cages' becomes 'pens' (I've seen this in another clue).
'o'+'pens'='OPENS'
(Other definitions for opens that I've seen before include "Unfastens" , "Allows access" , "Exposes, begins" , "Establishes" , "Tournaments for both professionals and amateurs" .)