One backing search for swimmer
I believe the answer is:
angelfish
'swimmer' is the definition.
(angelfish is a kind of aquatic vertebrate)
'one backing search' is the wordplay.
'one backing' becomes 'angel' (I can't justify this - if you can you should believe this answer much more).
'search' becomes 'fish' (fishing is a kind of searching).
'angel'+'fish'='ANGELFISH'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for angelfish that I've seen before include "Brightly-coloured swimmer" , "shark" .)