Frenzied swimmer circling continually (8)
I believe the answer is:
feverish
'frenzied' is the definition.
(thesaurus)
'swimmer circling continually' is the wordplay.
'swimmer' becomes 'fish' (fish is a kind of aquatic vertebrate**).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'continually' becomes 'ever' (I've seen this before).
'fish' placed around 'ever' is 'FEVERISH'.
(Other definitions for feverish that I've seen before include "Frenzied" , "highly charged" , "Restless with excitement or agitation - got ague?" , "in a tizz" , "hectic" .)