Marine animals accept eternally having higher temperature (8)
I believe the answer is:
feverish
'having higher temperature' is the definition.
I can't judge whether this defines the answer.
'marine animals accept eternally' is the wordplay.
'marine animals' becomes 'fish' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'accept' is an insertion indicator.
'eternally' becomes 'ever' (I've seen this before).
'fish' going around 'ever' is 'FEVERISH'.
Can you help me to learn more?
(Other definitions for feverish that I've seen before include "restless state" , "Frenzied" , "Restless with excitement or agitation - got ague?" , "hectic" , "Restlessly excited" .)