Sea harness restraining a jellyfish (7)

I believe the answer is:

medusae

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'sea harness restraining a jellyfish' is the definition.
I can't tell whether this defines the answer.

'sea harness restraining a' is the wordplay.
'sea' becomes 'med' (abbreviation of Mediterranean).
'harness' becomes 'use' (I can't explain this - if you can you should give a lot more credence to this answer).
'restraining' is an insertion indicator.
'use' placed around 'a' is 'usae'.
'med'+'usae'='MEDUSAE'

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I've seen this clue in The Guardian.

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