Crossword Genius

Porthole shutter, not at all heavy? (9)

Ross

I believe the answer is:

deadlight

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

'porthole shutter' is the definition.
'deadlight' can be an answer for 'shutter' (deadlight is a kind of shutter). I'm not certain of the 'porthole' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

I've seen this clue in The Times.
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