Restless swimmer always inside (8)
I believe the answer is:
feverish
'restless' is the definition.
(thesaurus)
'swimmer always inside' is the wordplay.
'swimmer' becomes 'fish' (fish swim).
'always' becomes 'ever' (synonyms).
'inside' indicates putting letters inside.
'fish' enclosing 'ever' is 'FEVERISH'.
(Other definitions for feverish that I've seen before include "Suffering from a high temperature" , "Febrile" , "Nervously excited" , "highly charged" , "Frenzied" .)