Swimmer's hi-vis jacket? (8,4)
I believe the answer is:
goldfish bowl
'jacket?' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?
'swimmer's hi-vis' is the wordplay.
I cannot really understand how this works, but
'swimmer' could be 'fish' (fish is a kind of aquatic vertebrate) and 'fish' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for goldfish bowl that I've seen before include "Situation lacking privacy" , "nowhere to hide" , "Pet holder; place without privacy" , "Situation open to observation" , "Place lacking privacy" .)