Unfriendly gulls moving south (5)
I believe the answer is:
stern
'unfriendly' is the definition.
'gulls moving south' is the wordplay.
I cannot quite understand how this works, but
'south' could be 's' (abbreviation) and 's' is found within the answer.
The remaining letters 'tern' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stern that I've seen before include "Grim, strict" , "Unrelenting at the back of the boat?" , "Authoritarian, back" , "Back end of a ship" , "Poop" .)