Artillery in working port (4)
I believe the answer is:
oran
'port' is the definition.
(I've seen this before)
'artillery in working' is the wordplay.
'artillery' becomes 'RA' (Royal Artillery).
'in' means one lot of letters goes inside another.
'working' becomes 'on' (on can mean working or activated).
'ra' put inside 'on' is 'ORAN'.
(Other definitions for oran that I've seen before include "African port" , "Algerian seaport" , "Algerian port" .)