Managed to enter Cooper's store (7)
I believe the answer is:
granary
'store' is the definition.
(granary is a kind of store)
'managed to enter cooper's' is the wordplay.
'managed' becomes 'ran' (I've seen this before).
'to enter' means one lot of letters goes inside another.
'cooper' becomes 'gary' (actor Gary Cooper).
'ran' put inside 'gary' is 'GRANARY'.
(Other definitions for granary that I've seen before include "Storehouse for wheat or corn, say" , "(Of bread) containing whole grains of wheat" , "Store for grain, etc" , "cereal 21 [STORES]" , "barn" .)