Party left public house with popular swimmer (7)
I believe the answer is:
dolphin
'swimmer' is the definition.
(dolphin is a kind of aquatic vertebrate)
'party left public house with popular' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'left' becomes 'l' (common abbreviation).
'public house' becomes 'ph'.
'with' says to put letters next to each other.
'popular' becomes 'in' ('in' can mean trendy).
'do'+'l'+'ph'+'in'='DOLPHIN'
(Other definitions for dolphin that I've seen before include "Sea mammal like large porpoise, friendly towards people" , "Small sociable whale" , "Sociable sea mammal, sacred in ancient Greece" , "Sea-mammal" , "Intelligent marine mammal" .)