Nonsense that's wacky, primarily rot! (7)
I believe the answer is:
twaddle
'nonsense' is the definition.
(informal term for nonsense or rubbish)
'that's wacky primarily rot' is the wordplay.
'primarily' suggests taking the first letters.
'rot' becomes 'addle' (I've seen this before).
The initial letters of 'thats wacky' is 'tw'.
'tw'+'addle'='TWADDLE'
(Other definitions for twaddle that I've seen before include "Pretentious or silly talk" , "Foolish speech/writing" , "Balderdash" , "Moonshine" , "Poppycock" .)