Direction in line astern (4)
I believe the answer is:
east
'direction' is the definition.
(compass direction)
'in line astern' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EAST' is hidden in the letters of 'line astern'.
(Other definitions for east that I've seen before include "English region" , "bridge-player" , "Asia" , "Towards the dawn" , "Bidder after North (bridge)" .)