Swimmer Don grabs drunken Phil (7)
I believe the answer is:
dolphin
'swimmer' is the definition.
(dolphin is a kind of aquatic vertebrate)
'don grabs drunken phil' is the wordplay.
'grabs' indicates putting letters inside.
'drunken' indicates an anagram.
'phil' is an anagram of 'lphi'.
'don' enclosing 'lphi' is 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Sea creature" , "member in school" , "Porpoise-like fish" , "Snall type of whale" , "Bottlenose creature?" .)