Herewith, a poem to make nonsense of (6)
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?" .)