Wild swimmer always dives in (8)
I believe the answer is:
feverish
'wild' is the definition.
(thesaurus)
'swimmer always dives in' is the wordplay.
'swimmer' becomes 'fish' (fish is a kind of aquatic vertebrate).
'always' becomes 'ever' (synonyms).
'dives in' indicates putting letters inside.
'fish' going around 'ever' is 'FEVERISH'.
(Other definitions for feverish that I've seen before include "highly charged" , "frantic" , "Suffering from a high temperature" , "Frenzied" , "restless state" .)