Three keys in sack (3)
I believe the answer is:
bag
'sack' is the definition.
(I know that sack can be written as bag)
'three keys in' is the wordplay.
'three keys' means three substitutions for 'key'.
'key' becomes 'b' (eg B major, musical key).
'key' becomes 'g' (musical key).
'key' becomes 'A' (musical key).
'in' is an insertion indicator.
'b'+'g'='bg'
'bg' placed around 'a' is 'BAG'.
(Other definitions for bag that I've seen before include "Secure" , "Container for holding or carrying" , "Reserve (colloq.)" , "Flexible container for shopping" , "Gladstone, for example" .)