Search a new set before swimmer (9)
I believe the answer is:
angelfish
'swimmer' is the definition.
(angelfish is a kind of aquatic vertebrate)
'search a new set before' is the wordplay.
'search' becomes 'fish' (fishing is a kind of searching).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'set' becomes 'gel' (both can mean to harden).
'before' says to put letters next to each other.
'a'+'n'+'gel'='angel'
'fish' after 'angel' is 'ANGELFISH'.
(Other definitions for angelfish that I've seen before include "Brightly-coloured swimmer" , "shark" .)