Pen and four keys (4)
I believe the answer is:
cage
'pen' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cage that I've seen before include "Structure of bars or wire" , "he used to write down scores" , "Barred enclosure for animals" , "Nicolas - - (films)" , "In which birds or animals are kept" .)