Crossword Genius

Let drop when draught contains iodine, sulphur and chlorine (8)

Ross

I believe the answer is:

disclose

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

'draught contains iodine sulphur and chlorine' is the definition.
I can't judge whether this defines the answer.

'let drop' is the wordplay.
'let' becomes 'disc' (I can't justify this - if you can you should believe this answer much more).
'drop' becomes 'lose' (I've seen this in other clues).
'disc'+'lose'='DISCLOSE'

'when' is the link.

Can you help me to learn more?

(Other definitions for disclose that I've seen before include "Divulge or expose" , "Divulge, reveal" , "Announce" , "Sold ices (anag)" , "Lay bare" .)

I've seen this clue in the Sydney Morning Herald.
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