Make incomprehensible poem after being regularly crazy in the head (6)
I believe the answer is:
encode
'make incomprehensible' is the definition.
'poem after being regularly crazy in the head' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem**).
'after' means one lot of letters go next to another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in the head' suggests taking the first letters.
The initial letter of 'crazy' is 'c'.
The alternating letters of 'being' are 'en'.
'en'+'c'='enc'
'ode' put after 'enc' is 'ENCODE'.
(Other definitions for encode that I've seen before include "Put (message) into ciphers" , "transform with substitutions?" , "Put into secret writing" , "Put into cipher" , "Encrypt" .)