Fish remain outside plastic ring (8)
I believe the answer is:
stingray
'fish' is the definition.
(type of fish)
'remain outside plastic ring' is the wordplay.
'remain' becomes 'stay' (synonyms).
'outside' indicates putting letters inside (some letters go outside others).
'plastic' is an anagram indicator (plastic can mean easily moulded).
'ring' is an anagram of 'ingr'.
'stay' going around 'ingr' is 'STINGRAY'.
(Other definitions for stingray that I've seen before include "Fish with venomous tail" , "TV show" , "swimmer" , "Dangerous fish" , "Flatfish with barbed spine" .)