Writer put in cage (3)
I believe the answer is:
pen
This clue is a double definition.
'writer' is the first definition.
(something which writes)
'put in cage' is the second definition.
(pen can mean to put in a cage)
(Other definitions for pen that I've seen before include "Put down" , "Pound" , "Writing instrument" , "See 20" , "'The ... is mightier than the sword'" .)