Called uninitiated mischievous swimmer (9)
I believe the answer is:
angelfish
'swimmer' is the definition.
(angelfish is a kind of aquatic vertebrate)
'called uninitiated mischievous' is the wordplay.
'called' becomes 'rang' (called on the phone).
'uninitiated' means to remove the first letter.
'mischievous' becomes 'elfish' (synonyms).
'rang' with its first letter taken away is 'ang'.
'ang'+'elfish'='ANGELFISH'
(Other definitions for angelfish that I've seen before include "Brightly-coloured swimmer" , "shark" .)