Marine creature's head swallowed by old fish (7)

I believe the answer is:

octopod

I'm a little stuck... Click here to teach me more about this clue!

'marine creature's' is the definition.
'octopod' can be an answer for 'creature's' (octopod is a kind of animal). I am unsure of the 'marine' bit.

'head swallowed by old fish' is the wordplay.
'head' becomes 'top' (synonyms).
'swallowed by' indicates putting letters inside.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'fish' becomes 'cod' (type of fish).
'o'+'cod'='ocod'
'top' put into 'ocod' is 'OCTOPOD'.

Can you help me to learn more?

(Other definitions for octopod that I've seen before include "Eight-legged creature" , "Eight-armed/footed" , "mollusc" , "Eight-limbed creature" .)

I've seen this clue in The Guardian.

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