Frantic always in the midst of swimmers (8)
I believe the answer is:
feverish
'frantic' is the definition.
(thesaurus)
'always in the midst of swimmers' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'in the midst of' is an insertion indicator.
'swimmers' becomes 'fish'.
'ever' put within 'fish' is 'FEVERISH'.
(Other definitions for feverish that I've seen before include "frantic" , "restless state" , "highly charged" , "Febrile" , "Frenzied" .)