A small seabird to ship’s rear (6)
I believe the answer is:
astern
'rear' is the definition.
(I have seen 'rear of ship' mean 'astern' so perhaps 'rear' could also mean 'astern')
'a small seabird to ship's' is the wordplay.
'small seabird' becomes 'tern' (I've seen this in another clue).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'ship's' becomes 's' (this might be a standard abbreviation of which I'm unaware).
'tern' after 's' is 'stern'.
'a'+'stern'='ASTERN'
Can you help me to learn more?
(Other definitions for astern that I've seen before include "Nautically behind" , "Behind at sea" , "Behind (a boat)" , "State of S central US" , "Toward the back of a boat" .)