Crossword Genius

Herewith, a poem to make nonsense of (6)

Ross

I believe the answer is:

encode

'to make nonsense of' is the definition.

'herewith a poem' is the wordplay.
'herewith' becomes 'enc' (abbreviation for 'enclosed' as in letters).
'a poem' becomes 'ode' (ode is a kind of poem).
'enc'+'ode'='ENCODE'

(Other definitions for encode that I've seen before include "Put into secret symbols" , "Encipher" , "Put into cypher" , "Make cryptic" , "transform with substitutions?" .)

I've seen this clue in The Independent.
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