Old seal swims in gulf ingesting lobster heads (5)

I believe the answer is:

sigil

'old seal' is the definition.
(historical term for a seal or symbol)

'swims in gulf ingesting lobster heads' is the wordplay.
'heads' indicates taking the first letters.
The initial letters of 'swims in gulf ingesting lobster' is 'SIGIL'.

(Other definitions for sigil that I've seen before include "Sign" , "Seal" .)

I've seen this clue in The Independent.

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