Barrel that is holding feline or marine invertebrate (8)
I believe the answer is:
tunicate
'marine invertebrate' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'barrel that is holding feline' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'holding' is an insertion indicator.
'feline' becomes 'cat' (cat is a kind of feline).
'ie' enclosing 'cat' is 'icate'.
'tun'+'icate'='TUNICATE'
'or' acts as a link.
Can you help me to learn more?
(Another definition for tunicate that I've seen is " sea creature".)