Admiral, primarily unsmiling, is back on a ship? (6)
I believe the answer is:
astern
'back on a ship?' is the definition.
(nautical term)
'admiral primarily unsmiling' is the wordplay.
'primarily' suggests taking the first letters.
'unsmiling' becomes 'stern' (I've seen this in another clue).
The initial letter of 'admiral' is 'a'.
'a'+'stern'='ASTERN'
'is' is the link.
(Other definitions for astern that I've seen before include "Towards rear of a vessel" , "Towards the rear of a ship" , "Toward the tail of a vessel" , "Behind at sea" , "Behind (naut.)" .)