Dock device for mooring gutted very small swimmer (7)
I believe the answer is:
anchovy
'swimmer' is the definition.
(anchovy is a kind of aquatic vertebrate)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for anchovy that I've seen before include "Small fish of herring family, nice on toast" , "Small strongly-flavoured food fish" , "Small, salty-tasting fish" , "Tiny fish, usually canned" , "Small fish used in making sauce, paste etc" .)