Imprison writer (3)
I believe the answer is:
pen
This clue is a double definition.
'imprison' is the first definition.
(to pen in is to imprison)
'writer' is the second definition.
(something which writes)
(Other definitions for pen that I've seen before include "'The ... is mightier than the sword'" , "Swan for the writer" , "Shut up" , "Small enclosure" , "International Association of poets, playwrights etc" .)