Night-flier circling round ship (4)
I believe the answer is:
boat
'ship' is the definition.
(I know that ship can be written as boat)
'night-flier circling round' is the wordplay.
'night flier' becomes 'bat' (I've seen this before).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'round' becomes 'o' (o is a circular letter).
'bat' placed around 'o' is 'BOAT'.
(Other definitions for boat that I've seen before include "Small ship" , "Water craft" , "Small vessel" , "Floating vessel" , "Vessel for water travel" .)