Naval bird? (4)

I believe the answer is:

wren

I'm a little stuck... Click here to teach me more about this clue!

'bird?' is the definition.
(I know that wren is a type of passeriform bird)

I can't explain the remainder of the clue.

(Other definitions for wren that I've seen before include "Tiny bird" , "serving girl" , "Member of the former Women's Royal Naval Service" , "seafarer" , "Small bird (in the Navy?)" .)

I've seen this clue in The Mirror.

Want a hint initially instead of a full solution? Install my app