Detectives chase sailor to a police box (6)
I believe the answer is:
tardis
'a police box' is the definition.
(Dr Who time machine)
'detectives chase sailor' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'chase' means one lot of letters go next to another.
'sailor' becomes 'tar' (slang term for a sailor).
'dis' after 'tar' is 'TARDIS'.
'to' acts as a link.
(Other definitions for tardis that I've seen before include "Famous time machine" , "Dr Who's time machine" , "Doctor Who's ship" , "Time travelling craft" , "Time-travelling box" .)