Crossword Genius

Swear that sulphur is included in the remedy (5)

Ross

I believe the answer is:

curse

'swear' is the definition.
('curse' can be a synonym of 'swear')

'sulphur is included in the remedy' is the wordplay.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'is included in' is an insertion indicator.
'the remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
's' placed into 'cure' is 'CURSE'.

'that' acts as a link.

(Other definitions for curse that I've seen before include "Invoke evil upon" , "Oath or a call for evil to come upon person" , "Blasphemy" , "Execrate" , "Malediction or expletive" .)

I've seen this clue in The Telegraph.
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