Weapon overturned by unknown heavy (6)

I believe the answer is:

drowsy

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'heavy' is the definition.
(thesaurus)

'weapon overturned by unknown' is the wordplay.
'weapon' becomes 'sword' (sword is a kind of weapon).
'overturned' says the letters should be written backwards.
'by unknown' becomes 'y' (algebra. I am not sure about the 'by' bit.).
'sword' reversed gives 'drows'.
'drows'+'y'='DROWSY'

Can you help me to learn more?

(Other definitions for drowsy that I've seen before include "Dull" , "Sleepy, somnolent" , "Torpid" , "Comatose" , "Half asleep" .)

I've seen this clue in The Times.

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